What is this button on my MacBook Pro Touch Bar?
That looks like it is from BetterTouchTool. You can configure it in the settings for BTT:

https://github.com/fifafu/BetterTouchTool/issues/1033


  fifafu: You can disable it in the Touch Bar settings in BTT (go to the Touch Bar tab, then click the "General Touch Bar settings" button.
  This may require a restart of BTT to become effective.