Unless we are talking about some very very early days, it has always been opt-in. Or rather, I have been installing Brave since ~2017 and I’ve never had Brave Rewards be on by default.
You’ll get notification badges on the Brave Rewards URL bar icon, but it isn’t active until you click on it and go through the BAT onboarding.
The Brave way in general seems to be "icon for function visible by default, function itself has to be turned on separately". So there's a bunch of clutter at first, but not stuff like say, Edge just turning history sync on even if you've previously turned it off.
You’ll get notification badges on the Brave Rewards URL bar icon, but it isn’t active until you click on it and go through the BAT onboarding.