Brave has perverse incentives to discontinue it because of their BAT crypto business model that rewards looking at ads.

Unfortunately even the fully open source Firefox isn't immune to the pressure from the advertising industry, with all their Google funding and their purchase of anonym.

You have no idea how BAT ads work in brave, do you?

I do, but even though they're not in the webpage itself and are as such not affected by the adblocker, brave still has an interest in the advertising industry. Many if not most of their advertising clients would use regular internet ads as well.

have you consider the possibility that... it is just too much work to merge/port the code when upstream is actively breaking them?

What's the big deal? Brave's native adblocker works pretty well.

For me it's uBlock origin or bust :)

I have a lot of custom rules in it too.

Fair enough, to each their own. I don't know if it's configurable at a file level but in the Brave settings you kind of can add your own rules. And you do have a point if the inability to use custom rules in private mode is enabled. It's pretty pernicious how they disabled that.