Google made breaking changes in protobuf (see e.g. here), in the generation and parsing of _pb2.py generated files.

Re the python protobuf package, the 3.20.1 release is followed by the 4.21.1 release.

AFAICT protoc >=3.19 generates _pb2.py files in the new format, while older protoc generates the old format.
AFAICT

• protobuf 3.20.x is able to parse both the old and the new format,
• protobuf<3.20 can only parse the old format,
• protobuf>=3.21 (i.e. protobuf>=4) can only parse the new format

We rely on protobuf to parse _pb2.py files at runtime:

• directly for paymentrequest_pb2.py
• indirectly via bitbox02 (hww plugin)
• indirectly via keepkey (hww plugin)

• Currently, our _pb2.py file(s) is in the old format, and we require protobuf>=3.12,<4.
  • see New protobuf release breaks electrum #7833
  • and 52b7388
• bitbox02 has _pb2.py file(s)
  • for bitbox02<6.1.0, in the old format
  • for bitbox02==6.1.0, in the new format
    • and it requires "protobuf>=3.21" (i.e. "protobuf>=4")
      • related PR
      • would be nice if they relaxed this to accept 3.20.x (done in PR, thanks!)
  • for bitbox02>=6.1.1, in the new format
    • and it requires "protobuf>=3.20"
• keepkey has _pb2.py file(s) in the old format
  • I've asked them to regenerate in new format and publish a new release
    • see issue