XOFF notice at edge connection
This Exit relay operator just informed us (on #tor-relays) that their Exit has seen this at least 979 times (duplicates hidden):
Request to start reading on an edgeconn blocked with XOFF
https://metrics.torproject.org/rs.html#details/B916FA873E00A1A1084DBA40DD767FB659BEA367
Also appears to be discussed in: http://eweiibe6tdjsdprb4px6rqrzzcsi22m4koia44kc5pcjr7nec2rlxyad.onion/tpo/core/tor/-/issues/40612#note_2806954