What happened to Blender's PTEX branch?

Was it really?

No

Too Hardware hungry. Not human-readable (and fixable in Photoshop).
UDIM’s is established as (the) standard for VFX industry - it does the job, nobody seems to complain.

f6c

At least for Blender users - this statement is true. There is no way to create Ptex in Blender, you need Mari or 3D Coat for that, and there is no way to render Ptex, as far as i know.
Maybe with Octane?