Not Fediverse, but an example of what works:
Quora used to have a fantastic crowd-edited question topic ontology/taxonomy for feed shaping with thousands of disambiguated topics interlinked in a graph/tree structure, but they enshittified the system away 2023, and now it’s all LLM misunderstandings of what the question topics should be. The bot thinks a question is about apples when the question is a mathematics word puzzle; “How many apples…”. The excuse for AI tagging was “tags can be abused” when in reality the crowd corrected the abuses quickly and reported the offenders. With the new AI tagger, my feed turned into viral trash from Quora-celebrities, mostly about current news.

They never made the obviously needed features of:

• being able to topic tag answers, not just questions (for when the question is general and the answer is specific, like “What do you think everyone needs to know in 2025?”).
• the ability to follow a topic from a user, because most people write about many topics, so a follow-a-user brings uninteresting topics to my feed, but not all people write well about that topic I want to follow, so I’d need to follow that topic from good writers only.

PieFed is a 100% for this.

Topic-focused puts you into the Threadiverse, where there are mainly 3 options: Lemmy, Mbin, and PieFed. I haven’t looked at nodeBB but that’s a fourth up-and-comer. (Also people expect flarum to gain ActivityPub support but it currently lacks it.)

Mbin’s major claim to fame is combining the topic-focused Threadiverse with the user-centric Mastodon like sharing, which sounds like not what you want… although it does have hashtags, and yet iirc only on the Microblogging side?

A very few - and unfortunately no longer maintained - Lemmy apps have some of what you want, but it is not worth what you would lose out on by doing so.

PieFed has perfectly what you have asked for. Hashtags sit on top of regular communities, so it is not either-or but rather both capabilities at the same time. And while I don’t know if you can block a particular hashtag (that feature should be added, if not, it’s a great idea!), the concept of keyword filtering (regardless of hashtags) can not only block out all of such content, but there’s even an option to only block out some, if you would rather, so that additional level of choice is nice. The only catch is that app support is experimental at best, so make sure to use the web browser view, at the very least to set up your account with the blocking that you want to see associated with your account, even if you then use an app for just daily browsing.

As Rimu (inventor of PieFed) already said, PieFed has numerous other features that you will fall in love with as well - e.g. categories of communities, which are both customizable and user shareable (so you can create a curated one if you like, and then share with literally everyone housed on the same instance, but there are pre-defined ones so that you do not have to) and many other features lacking on Lemmy such as user and post flairs, ability to hold polls, and just an absolute ton more behind all of that. It even goes further in terms of features than Reddit does, e.g. combining all the comments across all reposts of a particular OP into one view, to help deal with the fragmentation inherent in an implementation of the ActivityPub protocol i.e. the nature of the Fediverse trends towards fragmentation so this helps counter-balance that.

Happy explorations!

On Lemmy, you might find it overwhelming by looking at the all communities feed and then blocking those you don’t like. Instead subscribe to the ones you want and then just view the subscribe feed.

Personally, I use all to find more content and block communities I don’t want. I then just jumpnjnto subscribed occasionally of there is too much junk.

It’s a shame that there aren’t finer controls, like to ramp up and down communities rather than just block or subscribe. Some communities, I wouldn’t mind seeing their popular posts but I don’t want to say the hordes of junk posts. Only option is to block.

PieFed does that. See https://piefed.social/feeds which are basically collections of Lemmy communities that you can join or leave with 1 click.

aren’t the communities on Lemmy basically topics? why would that be overwhelming, you can easily select what you want?

You would love kbin.social.

It combined posts from Lemmy and Mastodon, allowing to browse and sort both of these like on Reddit. Mastodon toots were matched to respective magazines (like Lemmy/Piefed communities) based on their hashtags. And multireddit-like feature (called Collections there, and recently reimplemented in federated form by PieFed as Feeds) allowed any user to combine several magazines, Lemmy communities and Guppe groups in a single view.

All of these has been inherited by Mbin (a /kbin fork), outside of Collections/Feeds.

Largest instances are https://fedia.io and https://kbin.earth. There are much smaller, than flagship /kbin instance have been, though, so both will be aware of less posts…

You don’t need a “platform”, you need a Fediverse indexer + search engine.

Piefed maybe?

I like kbin, think of it basically like if someone took lemmy and mastodon and made one platform that could use both. there is only one mobile app but it’s pretty good imo.

Mastodon lets you follow and block hashtags. That’s how I got my feed going when I started.

pumpkindrama@reddthat.com NodeBB supports topic based following, because it’s a forum and that’s literally how it was done way back then.

You can follow tags as well.