There is not much need for it here anymore at least in this implementation.
The whole text engine have been rewritten.
You can instead reply and contribute to this thread ;
The good news is that the revamped text tool will be in Krita 6.0 which is now in beta 2.
it maybe just one more beta, and maybe a few release candidate before the full release.
You can test the new text tool in Krita 6.0 and check if the Justify is now working if not feel free to reply there, it was started by Wolthera who is the main developer of this specific feature.