@Saoiray: good point! 
In the meantime, I’ve found the solution for getting rid of that banner! 
In Terminal, just type defaults write com.brave.Browser SuppressUnsupportedOSWarning -bool true
Yippee! 
At least we can still use Brave until MacOS versions before 10.15 are dropped.
I understand all Chromium browsers are concerned, I’m nevertheless very sad as Brave has become my favourite browser (aside Firefox) in the past 2 years.
Thank you very much indeed for the Terminal hack.
Banner has been banished!
I’m very happy if it helped! 
I’ve found it in a Chrome discussion, the “com.brave.Browser” part is different for each chromium browser but the rest of the command is the same — it actually writes the needed flag in the com.brave.Browser.plist in ~/Preferences.
I am thinking of upgrading the old (late 2015) iMac to Monterrey, but am on Intel chip (i7).
Would the upgrade really slow me down so much? I have 32Gb of RAM.
Apologies if deviating somewhat from OP’s topic.
Kind regards, A