Over the years Nabble has changed several times. Originally it was a service that operated commercially.
Once it was funded through a premium support service at a cost. It has also been funded by ads. For a period you could pay to have these removed.
A couple of years back Franklin, the owner, closed down all the servers bar one and continues to operate the remaining server at entirely his own expense, as he wants to maintain a couple of forums of his own.
Obviously, his interest in maintaining the server could stop at any time hence my personal recommendation that users consider moving to another service.
It used to be effectively impossible to transfer a forum's data to another platform, but over the last few months a way of generating XML files containing a forum's data has been developed - but it will need considerable technical expertise to achieve a transfer.
I hope that helps you better understand the position..
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.