AZPainter: Simple Drawing Software

Project information:
AzPainter is a full color painting program for Linux. It’s a lightweight tool with tablet pressure support, and provides basic functions such as selection, layers, filters. you can use it for painting, image editing, or pixel art making.
(Description taken from Alternativeto.net)

AzPainter itself is more of clone to Windows-only PaintToolSAI, a painting software widely used in Japan.
It has features unique to them, even software like Krita or any open source other alternative doesn’t have yet, like per-preset hotkeys.

Name: AzPainter
Homepage: (Linux) AzPainter (author official site, Japanese only) | http://azpainter.osdn.jp/ (OSDN, English) | Azel / AzPainter · GitLab
License: GNU GPL v3
Upstream has been contacted: Yes, via email

Update regarding upstream:

Azel, the developer is not interested to maintain the flathub release, but he has no problem for any third party to redistribute AzPainter on Flathub.