The issue has already been reported on nanopb’s boards:
Apparently part of the problem may be some assumptions on pio’s end? My attempt to lock it to an earlier version explicitly as:
lib_deps = nanopb/Nanopb@0.4.5
or even something as old as 0.4.1
does not seem to change anything, it is still trying to get 0.4.6.4 and fails in the same way as in the linked ticket.
Is there currently no way to use nanopb with pio without some brute force contingencies?