I didn't notice the missing log message because apparently I only checked this in a live environment from an editable install where all plugins get loaded at once like before, unlike in non-editable installs.

This bugfix justifies another patch release after merging. Will publish 6.6.2 later today...

$ pip install -q .
$ streamlink -V
streamlink 6.6.1+3.g21ef70bc
$ streamlink twitch.tv/smoke
[session][info] Plugin twitch is being overridden by /home/basti/.local/share/streamlink/plugins/twitch.py
[cli][info] Found matching plugin twitch for URL twitch.tv/smoke
Available streams: audio_only, 160p (worst), 360p, 480p, 720p60, 1080p60 (best)