Depends what kind of income you are looking for and how you want to do it...
I've paid for a good few extensions myself, even when they are available as free.
There are sites like
https://www.canidev.com/ (I bought and use a lot of theirs, and I've even bought some I
don't use, because they are so cheap and I want to support them). Then I've also seen
https://www.dmzx-web.net/ and
https://phpbbdev.space/ where they use more of a "subscription" model and they have a lot of stuff for free and a couple of premium ones (typically the ones that have the potential of making
you money, like a PayPal subscription extension).
Then there is
https://kasimi.net/ for example, also a subscription but for extensions that expand the use of mChat which is a hugely popular free chat extension. Kasimi hasn't been active for a long time but the donations keep coming (still pocket money, but it adds up I guess).
There are people selling extensions separately too:
https://github.com/senky/httpproxy (links don't work anymore, but I got them back in the day and they still work), etc etc.
And then there are these people:
https://sitesplat.com/phpBB3/ who built an entire community and client base around their theme.
It will definitely depend on a lot of goodwill. It's open source, everything is Apache license (I think?) so it's very hard to protect yourself from copying stuff, and people can even take your code and make it their own, I've seen that happen plenty of times. It's a little bit the nature of open source, but also there is quite a large market for a solid and useful extension.
Not a terribly clear yes/no answer, but hope it helps...