There are a few, Kolourpaint is pretty much an MSPaint clone for KDE [1] and Paintbrush is the macOS equivalent [2]. There is also Gnome Paint [3], but hasn't seen any update in years. I suppose the closest would be Pinta [4], although that is slightly above MS Paint on the complexity level (probably more comparable to Paint.NET than MS Paint).
> I'm not sure how I feel about it being web-based
Personally i find it amusing and impressive that this implements a Win95-like GUI (i love Win95's GUI :-P) but at the same time the actual editing feels very slow - painting something takes almost a second to update with the brush being very jumpy, reminding me using ZSoft's PhotoFinish on Windows 3.1 on my 4MB 386:-P.
It is also nice that it has implemented the select+shift+drag brush feature that is ignored by almost every other clone i've seen over the years.
[1] https://www.kde.org/applications/graphics/kolourpaint/
[2] https://paintbrush.sourceforge.io/screenshots/