In Defense of Timelines
On Mastodon, there are 3 main timelines: Home (everyone you follow), Local (everyone on your instance), and Federated (Local + a whole lot more). Local runs slower or faster depending on how big/active your instance is. Home you control based on how many people you follow and how active they are. Federated is sort of a product of the size of your instance because it's partially based on the people that anyone on your instance follows.
There's also instances, which can have a specific purpose (I'm on one of those), or be general purpose. Local is far more useful on the former, but Eugen, the developer of Mastodon, is on the latter. He's mentioned getting rid of the Local timeline (as he already has done on the iOS app because of Apple). If you're on a general purpose instance or have been around a while, it might not be a problem. However, if you're new or on a specialized instance, this is a massive problem.
Without finding people via the Local and Federated timelines, it's difficult to find people to follow to populate your Home timeline! This is especially a problem for new users and people on specialized instances, of which Eugen is neither.
This is not about the iOS app but Mastodon in general. He can do what he likes with the iOS app as he's not got a near-monopoly on mobile access to Mastodon.
A quick analogy for those not on Mastodon
Let's take another site that doesn't entirely follow the same model but it's close enough: Reddit.
You log in and you see a bunch of posts from subs you've subscribed to. That's the Home timeline. Local would be the equivalent of the home page of one subreddit. Federated would be /r/all. Let's say the devs there propose that they're getting rid of the front pages of every subreddit and /r/all. If you want to find anything beyond what you're already subscribed to, you're going to have to hunt for it.
I imagine you wouldn't find that to be a good user experience because now you'd be unable to easily broaden your horizons.
New users and discoverability
A long time ago, (2009 or so), I joined Mastodon on mastodon.social, a general purpose, the biggest, and Eugen's home instance. There were about 300k people at the time but by default the Home timeline is near dead because the admin is the only person you follow. I didn't know anybody on there when I started. I started because I felt I'd been wronged by other social media.
So how do you find people to talk to? Well you can either tag along to conversations to which the admin is a part of or jump onto the local/federated timelines. I ended up doing the latter.
So what happens when they're gone? You're stuck with the former! You can't find other people and vice versa except via the admin. If you don't get on with the admin and have no one else to talk to, you leave! So much for a social network, eh!
Special purpose instances
Special purpose instances (as a whole) contain roughly as many people as general purpose ones, just scattered to thousands of different instances instead of the few big ones.
They may be based around a certain hobby, political ideology, religion, region, friend group, language, safe space for a group, or all sorts of other things. The rules of the instance may be stricter because the instance is dedicated to that topic and that topic alone. Some other instances may be hostile to the users of your group because of opinions that differ strongly from it. In this case, the local timeline is far more useful than the federated timeline. It's not just a means to find other people to talk to. It's a means of finding other people within the same group to talk to.
The federated timeline may be dangerous to some folks, generally those of marginalized groups. Instances intended for them tend to block heavily but it's like an antivirus, a perpetual game of cat and mouse, trying to block others that intend to do them harm. In that case, the users NEED the local timeline because the federated timeline could be fraught with danger.
Even on other instances where users don't need to exercise that level of caution, you may not want to go on about a random topic with a stranger on another instance without knowing if that's what they care about.
So why do this?
I hope it's a lack of perspective. Eugen has been the admin of a general purpose instance for at least 5 years, and it by itself is about 15% of the whole network, so his opinions would be colored by that. He is trying to customize the experience for those users. He considers each instance to be interchangeable, is trying to hide the differences between them, and trying to reduce barriers between communicating between them, as per his explanation:
The “local” timeline makes the concept of “you can follow anyone from any server” less concise because suddenly you do care about being on the same server as specific other people.
Some people have a very good reason to care about that! People on specialized instances choose to be there because they intentionally want to be with others of the same group. Dissolution of the local timeline makes it less convenient to talk to to others on the same instance. Instances specialize, and stop communicating with certain others, for a reason. Some instances have good reason to be very insular, whether because of their particular specialization or for the safety of their users. Removing that ability harms those users and doesn't help people reach out beyond their instance. They might do because they have no one else to talk to, but that's not "help". That's desperation.
New users also have a steep on-ramp onto the network unless they know someone already there. Removing the local and federated timelines turns that into a cliff face. People will leave instead of finding strangers to talk to, which is bad for the old metrics and the health of the network as a whole.
He states that it's an issue with "frustration either through language barriers, spam, or not safe for work content." If that's such an issue, then the admin should be the one in control of switching the timelines on and off for their instances. The admin/mods can't be in control of everything but if they have issues with moderation/rules of other instances, the admins already have the ability to block/silence/block media from those instances. Blocking the federated timeline would be pretty much a last-ditch effort when the report queue got too high.
Also finally, "Also has people mistaking it for geographical location". That might do with a name change to "Local Instance Timeline" or "Your Instance Timeline", but that's no reason to remove it.
So what now?
I can only hope that the idea isn't being seriously considered. If so, then the outcry over this might well change his mind. This isn't likely given past history for him to say "I'm doing what I think is right and screw the rest of you." There's already hard feelings regarding the idea of rich text in toots, which was shut down by him months ago.
If the timelines do get wiped out, the code can always be forked, as has been done a number of times. Local and federated timelines are just showing toots that already exist on the network and the code still lives in source control, so a fork can still maintain those features. I hope it doesn't have to come to that.