Funnily enough, PoW was a hot topic in academia around the late 90s / early 2000, and it’s somewhat clear that the autor of Anubis has not read much about the discussion back then.
There was a paper called “Proof of work does not work” (or similar, can’t be bothered to look it up) that argued that PoW can not work for spam protection, because you have to support both low-powered consumer devices while blocking spammers with heavy hardware. And that is very valid concern. Then there was a paper arguing that PoW can still work, as long as you scale the difficulty in such a way that a legit user (e.g. only sending one email) has a low difficulty, while a spammer (sending thousands of emails) has a high difficulty.
The idea of blocking known bad actors actually is used in email quite a lot in forms of DNS block lists (DNSBLs) such as spamhaus (this has nothing to do with PoW, but such a distributed list could be used to determine PoW difficulty).
Anubis on the other hand does nothing like that and a bot developed to pass Anubis would do so trivially.
Sorry for long text.
I don’t like it, but I just recently switched to Brave on Android coming from Firefox.
I’ve been using Firefox on Android for years, but the same problems kept bugging me. Tabs would instantly close after putting the browser in the background, effectively making it impossible to e.g. get the 2FA out of my Authenticator without restarting the login flow. Also sometimes the browser would right out refuse opening websites on fresh tabs.
This were likely problems with my OEM, but I could never get it fixed. There are some threads online claiming battery optimization, but I disabled everything possible.
So on Android, I’m stuck with Brave for now.