I don't think many people will care, as this only happens with some very specific settings, but I thought I report it anyway. I noticed because in devpi-server I wrote my own un-/serializer implementation for performance reasons and the tests still compared to the execnet implementation. Afaik Python2 strings were never used in devpi-server databases and the new implementation is in use since 6.2.0 released August 2021.

With pre 2.1.0:

from execnet.gateway_base import Unserializer
from devpi_server.fileutil import BytesIO
Unserializer(BytesIO(b'M\x00\x00\x00\x01aQ'), strconfig=(False, False)).load(versioned=False)
b'a'

With 2.1.0:

from execnet.gateway_base import Unserializer
from devpi_server.fileutil import BytesIO
Unserializer(BytesIO(b'M\x00\x00\x00\x01aQ'), strconfig=(False, False)).load(versioned=False)
'a'