I'm trying to stay ahead of the game when I can by making sure stuff is saved in more universally readable formats. For instance, I used to use Final Draft for screenwriting but gave it up in favor of applications like
(beat) and
Highland that work with a kind of specialized Markdown called Fountain -- so it's a plain text based format. I also made sure my back library of scripts was all saved out into PDF, so that if eventually I can't open an .fdx Final Draft file, it's not a problem. And then scripts aside the vast bulk of my writing is in Ulysses, which uses Markdown (again, plain text). I'm not at all worried about that being readable going forward
The things I currently worry about most are the contents of my Apple Notes app, which has no decent export options I'm aware of.