Well, there is Open Motif for those that feel like using it.

I meant OpenMotif as pjmlp mentioned. These days it seems to mainly receive patches to keep it working but it maintained API (and ABI where possible) compatibility even when it was more actively developed as it was a design choice, not a byproduct of not receiving updates. AFAIK it is how Lesstif managed to be a (mostly) drop-in binary compatible alternative to Motif before the latter became open source.