the service normally does not require registration or login, neither any kind of payment, and they don’t run ads.
however, their public instance, meet.jit.si, got restricted a year or so ago because apparently bad people were using it for bad reasons. now a single participant is required to log in with a google account or something else they offer.

there were also many public instances. but soon a lot of them closed too, and the public lists have disappeared, so I believe jitsi did not do what they did out of data hunger.
there’s still a few that doesn’t require any kind of account, if you’re interested I can send a link in PM. It’s better if you don’t publish it either.

oh and if it tells anything. if you have heard about matrix.org, they were using jitsi for calls for a long time. they have switched to an in-house solution to have better platform integration, but it was fine