You can download Full Brightness Toggle on CurseForge and Modrinth:
  CurseForge:   https://curseforge.com/minecraft/mc-mods/full-brightness-toggle
  Modrinth:      https://modrinth.com/mod/full-brightness-toggle
To keep a better overview of all mods, the issue tracker is located in a separate repository.
  For issues, ideas, suggestions or anything else, please follow this link:
    -> Issue Tracker
Because of the way mod loader files are bundled into one jar, some extra information is needed to do a PR.
  A wiki page entry about it is available here:
    -> Pull Request Information
Requires the library mod Collective.
   This mod is part of Serilum's Client Bundle mod.
Full Brightness Toggle is a client-side mod which allows players to instantly turn the brightness (gamma) to full with the press of a button. By default it's G, but this can be changed in Options -> Controls. Created after reading a request from someone with impaired eyesight, who needed this feature to play the game. Don't use it on servers where it's not allowed. When in doubt, ask first.
The feature in action:
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The keybind can be changed in the in-game options menu:
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You may freely use this mod in any modpack, as long as the download remains hosted within the CurseForge or Modrinth ecosystem.
Serilum.com contains an overview and more information on all mods available.
Comments are disabled as I'm unable to keep track of all the separate pages on each mod.
For issues, ideas, suggestions or anything else there is the Github repo. Thanks!