So, Sai has for some indeterminable reason(proly cultural) a heavily encrypted file format. Wunkolo on github/gitlab has been trying to decipher this format and made 'libsai' with the efforts. We should see how difficult it is to get this working in Krita, so users can open their old painttool sai files.
From the 2019 sprint, @rempt and wolthera sat down to figure out how difficult it is to add the library. Boud came to the conclusion it'll take a bit of effort to get it working on Krita given libsai is 1. C++14, and 2. threw a bunch of errors when we tried to compile it as part of Krita.