No idea if this justifies having another patch release just yet, considering that the Twitch plugin is priority number one... I'll think about it. Neither Twitch's website, nor my Twitch GUI use uppercase channel names in any way, so CLI users of Streamlink can always use lowercase channel names as a workaround in 6.8.2.

$ ./script/test-plugin-urls.py twitch -r CHANNELNAME HasanAbi -i video -i /v -i clip
:: https://player.twitch.tv/?parent=twitch.tv&channel=HasanAbi
::  audio_only, 160p, 360p, 480p, 720p, 720p60, 1080p60, worst, best
:: https://www.twitch.tv/HasanAbi
::  audio_only, 160p, 360p, 480p, 720p, 720p60, 1080p60, worst, best
:: https://www.twitch.tv/HasanAbi/
::  audio_only, 160p, 360p, 480p, 720p, 720p60, 1080p60, worst, best
:: https://www.twitch.tv/HasanAbi/?
::  audio_only, 160p, 360p, 480p, 720p, 720p60, 1080p60, worst, best
:: https://www.twitch.tv/HasanAbi?
::  audio_only, 160p, 360p, 480p, 720p, 720p60, 1080p60, worst, best