SquarePay for WHMCS is built and maintained by JEANDRET, a hosting company that runs this payment module on its own WHMCS install, for its own customers. When Square changes something, it breaks our billing before it breaks yours — so we find it and fix it.
We built SquarePay because there was no working, supported way to take Square payments in WHMCS and let renewals charge automatically without card data ever touching your server. Card details are entered into Square's own hosted fields and exchanged for a token; WHMCS stores only that token, the card brand, the last four digits and the expiry date. That keeps your install in PCI DSS SAQ A scope and works correctly with local credit card storage switched off.
This is a focused product, not a side project: one module, done properly, supported by the people who depend on it themselves.