In the UK, you're right you can get that time off, but the person can also claim back all the money they make during it from HMRC, which they'd get around 90% of their already top wage.It's really up to the patrons in this case. If the sole artist (who I think we agree should not be temporarily replaced by someone else for the sake of consistency) gives birth, it's normal in most civilised contries to get parental leave. Where I'm living it's indeed quite normal to get at least a year. So it's understable that she took a real break for some time.
It takes 5 seconds to stop your contribution on patreon, so one would hope, that those people who think they didn't get what they gave their money for, have stopped giving long ago.
I'd think the people funding you to work on a game, which literally cannot progress because the artist isn't working, should not be taking funds for no reason. The developer can also pause their patreon, but they didn't.