Do you have any idea whether this is retroactive or going forward? Like can they claim the rights to everything I've ever posted, or only after the new user agreement?

yes, they can and they do.

It’s does explicitly say it does or doesn’t, so I would act on the assumption it is retroactive. They’re probably using everyone’s content already, so this new TOS would indemnify them.

I think we can assume they have already scraped everything ever posted on Twitter and are doing whatever they want with it (and LAION probably scraped the entirety of Twitter sometime before 2022 as well. Along with everything else.)