I can’t see anywhere why. Anyone knows?
I’m guessing this is linked to the author leaving Tweag, and thus not maintaining Ormolu anymore? Pure speculation
Ormolu seems to be in a weird place right now.
- On August 18th, Mark Karpov (the author) uploaded version 0.9.0.0.
- The next day, Mark published a blog post about the release.
- At some point,
https://github.com/mrkkrp/ormolustopped working, meaning it was removed or made private. - https://github.com/tweag/ormolu still works, but the latest release on there is only 0.8.1.1.
- On September 16th, Mark published another blog post about a new formatter called Tilia.
Trying to read the tea leaves, I’d say that Mark released a new version of Ormolu, then Tweag decided they’d rather maintain control so Mark released Tilia instead. I don’t know if that’s actually the case; it’s just my read of the situation.
More specifically about the GitHub action, you could use my Setup Ormolu action instead.
Yes, similar like Tweag took over the ownership of ormolu
Hey folks,
Mark recently left Tweag after many years and substantial contributions to Ormolu. We’re grateful for the work he put into it. Tweag continues to develop and maintain Ormolu at tweag/ormolu.
I can’t speak to why the separate run-ormolu GitHub Action was archived but the action taylor suggested seems like a good alternative! 
To add to this answer: Tweag repository has tag 0.9.0.0 off the main branch. In contains exactly the version that was uploaded to hackage, unfortunately commit history is not persisted.
The code is not on the main branch yet, as the library does not pass self tests (likely to the change in Diff package behavior), if so it’s resolved on hackage and main branch, but 0.9.0.0 adoption requires some time. As we want to ensure that nothing is broken.
sorry for the delay and we will provide an update once next version of ormolu will meet this world.