According to this --->
https://bitcointalk.org/index.php?topic=2385104.msg24371150#msg24371150I wrote the system so that I can easily add additional paid memberships in the future, but I might not ever do so. Not sure.
It looks like a modified library that was used to generate addresses but I think most of the modifications come from
https://btcpayserver.org/ ?!
In general, I think that the above project is open source and provides an option for everyone who does not want to strive to write code from scratch, and therefore @theymos is the one who can confirm the nature of the code.