Words have a meaning you know? "Discoverability" comes from "discover", which discribes an act of looking for something and not having something pushed into your field of view with minimal own effort.
Communities want more discoverability to get more members that post relevant things. This does the opposite and actively hides the specific community from potential posters while increasing the noise in the comments.
I think people really need to have some serious thought about the consequences of what they are asking for. These feeds, similar to algorithmic recommendations of commercial social media, increase engagement (a dubious metric, primarily interesting for advertisers) but not discoverability.
No, the problem is that people that have no relation to the community start commenting and getting into arguments.
Say for example a /c/anarchism gets added to a "politics" feed. And suddenly you have a bunch of people that have no clue (or even a pretty false idea) commenting on posts in the anarchism community because they think it is just another politics posts. Then others that are actual members of that community start getting into largely off-topic arguments with these commenters and when moderators step in you shortly after get complaints from people about being "censored for their totally valid opinion about politics" and so on.
Yes having that option more easily accessible would be much apprechated.
Once a community is known to an instance it is available via the search feature. Thus this really doesn't improve discoverability at all assuming the person adding it to the feed is already using the instance.
What it however does is moving the conscious choice of looking for and joining a community to an opaque follow feed button that makes someone subscribe to a lot of communities they know nothing about other than that someone else thought they somehow fit to a single word tag (and it is worse than hashtags on Mastodon as it is not the person making the post that adds them, but a totally unrelated 3rd party).
Yes the All feed has the same problem, but posts need to be significantly more popular for them to even register in the All feed. Thus most small communities currently fly under the radar of the All feed, and if they do get a popular post it nearly always becomes a moderation nightmare.
Hashtags on Mastodon have a similar problem, having given rise to the universally dreaded "reply guy" issue.
I think most people on Lemmy haven't really thought this through and what the implications of such a feature are once it becomes widely used.
And no, the one that is doing the opt-in is the person creating the feed without asking the community that is being forcefully opted-in. Giving them the option to veto that is better than having them realize that they have been opted into something they don't agree with by being flooded with trolls and off-topic comments.
If I don't misunderstand then you can only add communities to these feeds that are already known to your instance, thus I don't really see how this solves the federated discoverability issues which are ultimately due to instances not being aware of each other at all.
This will reduce the discourse quality significantly as it will bring in more drive-by comments from people not subscribed to the specific communities in question.
I hope there will be some way for communities to opt-out from this or maybe better require them to opt-in.
Charity is not the same as mutual aid anyways, even though I have also seen "mutual aid" requests on the Fediverse that were clearly asking for charity.
Matrix servers have the problem of highly variable resource use.
Basically if you only use it for some light chatting with friends and family and some niche topic public rooms it isn't very heavy.
But if any user of your homeserver joins any busy rooms or uses the bridges to join busy public Telegram channels or such, it will quickly outgrow the resources of a reasonably priced VPS.
Personally I would rather recommend you to set up an xmpp server, which can include a gateway to Matrix and other services, but architecturally is much more lightweight and has better mobile clients.
Igalia is currently working hard on making it easy to use Servo as an embeddable browser engine similar to how Chromium can be used.
The problems of doing that with Gecko, the browser engine that powers Firefox, is main reason why there are so few alternative browsers based on it.