There are apps like https://offpunk.net/ which are explicitly low tech (and even solar punk). But there are also apps, which just happen to be offline first (think PWA etc.). And things in between, like Syncthing. Some might be self hosted other maybe local or distributed applications.
What’s your favorite offline-first app or tool?
A key advantage of tools like obsidian is the local markdown data. So you can basically use this client until you don’t anymore. Then just move on, if you don’t use very specific plugins or something. So rather little risk here, in my opinion.
But other popular alternatives you might want to check out are logseq and Joplin.