I have been using QOwnNotes for about 6 months. It is cross platform, lightweight, extensible and a plain-text markdown note-taking program written in C++/Qt. It can integrate with Nextcloud. Installable via scoop on Window and apt on Debian† (after installing their apt key).

Author is quite responsive on GH issues.

† Also most major Unixes

Obsidian and a shared Dropbox/Google Drive/OneDrive folder?

I personally use Markor on Android and (neo)vim on Linux. I sync the note files using rsync (but syncthing is totally viable also).

It's probably not the solution you want but thought I'd mention it for others interested.

I use Joplin synced over Nextcloud.

Standard Notes is pretty good too

Depends on what method h use for note taking.

If it's not that professionally or for research purposes used then simple note is great.

If it's about research data then obsidian.

Also u will obviously find it difficult to find a good FOSS app for note taking that many of them won't support sync and collaboration.

Emacs + org-mode + maybe org-roam

Amazing features, can export to a bunch of common formats.

I came here to recommend Simplenote. Sorry it's not working for you anymore. I've loved it for years.

Oh, I know! Start carrying a pen and a tiny notepad. It's not multi-platform, but it will rarely undelete. /s

Affine.pro has potential but a couple months from cloud.

SiYuan is solid and FOSS (GPL)

I looked into it once but there wasn't anything. I just stuck with Google Keep and it works great for small notes.