Aseprite used to be open source. And because it was licensed under the GPL, a license with copyleft, a fork could be created after it went closed source.
If you're on Linux, you can also install it via flatpak
There's also LibreSprite, a fork of Aseprite. Aseprite used to be under the GNU GPL but at some point it moved to a proprietary licensr. LibreSprite aims to provide the same functionality as Aseprite but still under the GNU GPL.