Primal and Damus, for example, both said in November 2023 that they would implement Outbox in 2024. They even were on a panel at Nostrasia discussing this. They're working on other things at the moment. Damus Notedeck will come with outbox by default. A while back Primal said they had begun taking steps for outbox.
Discussion
Thanks, any idea why other things have taken priority? I see Primal has put considerable effort into feeds and indexing, though outbox would seem a more fundamental and therefore pressing thing to get done. I'm assuming there's a strong counter-argument, as their team is clearly plugged in.