Earlier quoted context omitted.
Spoilers for a game that cannot be played in 2021 and can barely be watched on YouTube: Morpheus died in The Matrix Online and the filmmakers have stated the events in Online remain canon.
I don't know if that's a good excuse - Neo and Trinity both died in Revolutions but Keanu and Carrie-Ann Moss are back.
From the stand point of "Morpheus sacrificed himself for Neo", that's a good story, even if very few players were around for the game event where that happened "on screen". I think its a pretty decent excuse for "Morpheus isn't involved in Matrix 4" even if I do hope that they find a way somehow to tell that story in a better way than "well if you look for the right YouTuber you might get an okay retelling of what playing The Matrix Online was like" and/or compilations of the live events cutscenes. (Related aside: Owning what was once the developer Monolith, Warnermedia should own all of the assets of The Matrix Online, except maybe remaining clauses in publishing contracts with Sega/Sony/what is today Daybreak. I wonder if some sort of F2P revival might have been an interesting marketing plan for Matrix 4.)
I don't have any direct theories on Trinity's return. There's rumors about where Matrix Online's storyline was supposed to go before the game was shutdown, of course, but rumors of a storyline and events that didn't even happen are certainly non-canon. I am of course very curious to see how they address it.