[Macs only — spoiler, only tested with numeric keyboard]
SIII dude here always played on mac, starting to have that issue. On my macbook pro never had that issue but since I bought my iMac... man that stress me so much. Today (as of writing this) I had it with league knowing whats best for my brightness value - I have a 27" iMac... it burns my eyes out on max - so seeing this topic (https://boards.na.leagueoflegends.com/en/c/help-support/eZdwXvv0-brightness-fullscreen-mac) made me want to solve the problem.
And surprise surprise! in 10 min I figure it out. But unfortunately I think this will only work on numeric keyboards.
[zombie-nunu-tears]
In fact as Broporo says (on the archived post) is a keybinding black magic. But if go to (bare with me in the translation):
_system prefs > keyboard > shortcuts > screen _(in the side defs) you will see, or can add the following:
https://imgur.com/a/t8QSK6M
And you can assign - i guess - from F13 to F19, the brightness up or down. Just tested it and it works.
At least for me no more burned eyes!!
[slayer-jinx-wink]
Hope this will help at least mac os gamers with numeric keyboards ;) Such niche ahah