See here:
If this option isn’t available anymore (alluded to in this post)
Then I’m afraid there isn’t any way to do this anymore. Potential way to circumvent this (but I can’t advise this) is (Assuming win10):
-Copying your user data folder located here C:\Users[user name]\AppData\Local\BraveSoftware\Brave-Browser\ as a failsafe
-Pasting it elsewhere (like desktop or Documents)
-Renaming the User Data folder in appdata folder
-Uninstalling Brave
-Reinstalling an older version (here is a version with the option I mention above still available https://github.com/brave/brave-browser/releases/tag/v1.50.114; Grab BraveBrowserStandaloneSetup.exe. If it still updates during the setup process, I’m sorry I’m not able to help further)
-If this is successful, use windows to search “Task Scheduler”, open that, go to “Task Scheduler Library”, and disable the two BraveUpdater tasks.
-Open Brave and do the brave://flags/ thing mentioned above, then close Brave
-Delete all the files in the new User Data folder, then copy the contents of the renamed User Data except the “Last Version” file over to the new folder
Expected result should be the “old” Brave you were using. If this isn’t the case, I apologize. Again, for browser safety reasons, I can’t actually advise this. This is only an option if you decide to personally go for it. Only other option is to move over to another browser if able. You will either have to look up how to import data to a new browser, or someone else would have to let you know how if that’s even possible.