For Linux there is Pinta, which is a very similar UI to pdn. It works well enough much of the time but I've found it way more unstable than pdn (and without the plugin ecosystem).
I'm not sure what it is on my machine(s), but I've found Pinta to be very unstable. It'll crash to desktop without warning in the middle of random operations. If it would just work reliably it's the paint app I want to use.
There have been issues with the packaging of the app. I can't remember precisely, but I do remember some crashes due to mismatches of some expected library version in Debian family. I'm using the version on Flathub which seems to work, and while I dislike some of the recent UI decisions like the ColorPicker is now worse (apparently enforced on them via GTK changes), I do still like Pinta.
Pinta actually uses an older version of the paint.net image manipulation functionality from when Paint.net was open source, with a rewritten UI (in gtk# if I recall correctly, compared to WPF for Paint.net). However, that new UI is significantly inferior to Paint.net's. One example - last time I tried it, the "marching arts" to highlight selections was sized in terms of pixels in the source image. It was always 1 source image pixel wide, rather than 1 screen pixel. This significantly hurt its usefulness for pixel art type situations.
