After reading you, @eapl.me@eapl.me, I’ll tell you my point of view.
In my opinion, a feed does not have to be equivalent to a timeline. A timeline is a representation of the feed adapted to a user. You may not be interested in seeing other people’s threads or DMs. But perhaps they are interested in seeing mentions or DMs directed at them. It is important not to fall into the trap. With that clarification…
I insist, this is my point of view, it is not an absolute truth: I don’t think extensions should be respectful of customers who are no longer maintained.
We cannot have a system that is simple, backwards compatible and extensible all at the same time. We have to give up some of the 3 points. I would not like to give up simplicity because it will then make it harder to maintain the customers who do stay. Therefore, I think it is better to give up backwards compatibility and play with new formulas in the extensions. I don’t think it’s a good idea to make a hash keep so much load: a hashtag, a thread and also a DM.

@andros@twtxt.andros.dev nothing stands still, I agree. I think current twtxt has surpassed the initial specification, while still being relatively backwards compliant/compatible but, for how long?

As for new extensions (DM, for example), they should be OK as long as those working on clients can reach an agreement on how to move forward. That has proven, though, to be a pickle in the past.