nd if you take away that concern then Brave is fully open source.
While I kinda agree, I personally call these "open source versions", as in they have less features than the paid versions. I don't see these as bad, this is cool, but when previously open sourced features become paid features I think that's really gay. Look at Gravitee for a good example. Most of their currently paid features were free features in their previous version, and anyone who doesn't like it can keep using the old, buggy, insecure one or pay at minimum thousands a month.
This isn't exactly what brave did, and they're not doing anything against the spirit of open source I guess, but it feels odd that they claim to be open source when all the features that make it different are actually closed source. Why not open source brave sync, is the code shit and they're afraid of everyone seeing the vulnerabilities? Are they afraid a competitor will do it better? Code to sync data between two clients isn't really rocket science, so I don't see an issue "secret work" wise, it's not like their competitors could suddenly get an edge.
Usually it's acceptable to not include api keys because they're to 3rd party services, and for many there's alternatives you can use e.g minio for s3, but this is all brave-built with no alternative. Want the features? Use it from us, or else.
For open source software to survive we need the devs to eat, so premium features are cool and you can't really just do hosting anymore with aws & etc eating that market, but there should be a bit more transparency in the advertising.
I like open source because features are reliable since I can have everything fully local and depending on me, not on some company that may or may not survive in the future. If the developer of your favorite open source software with a premium version you were depending on dies and there's nothing that has the exact same things you needed in one package, wouldn't that suck? Move on or diy I guess, if you're uerisimilitudo or similar you'll stoically not give a shit, but normal people care when stuff they have used for ages just disappears on them.