Is becoming a supporter a subscription?

No. You don't have to pay more than once if you don't want to. You can become a supporter at whatever level you feel comfortable with, and not pay again unless you want to show your appreciation (kinda like a tip jar).

After your supporter status expires, all of the advanced features that you had when you were a supporter will remain available permanently.

Of course, I hope that people choose to support at the 12-month level, and choose to renew their support in the future, but there's no need if you don't think it's worth it.