Yes, Krita is Open Source!
A look in the manual would reveal it:
As well as a look on the homepage:
Even the Repo, that you may need to develop your desired feature, states it:
BTW, things you may need, perhaps you’ll need the Krita Scripting School too:
https://scripting.krita.org/lessons/introduction
And the Plugin ‘‘Python Plugin Developer Tools’’, made by @KnowZero, you may need also:
In Krita, under ‘‘Help’’ → ‘‘About Krita’’ in the tab ‘‘License’’, is stated that Krita stands under ‘‘GNU General Public License’’ Version 3 (or any later Version), there you can learn about the rights you’re granted, also the license itself can be found there. The term Open Source isn’t used there, but the license says everything.
Something like some initiative would certainly be useful, but then you would already know that Krita is Open Source…
Michelist