s2Member Pro still uses the same code to interact with PayPal, there's a core integration that's already in the s2Member Framework, and s2Member Pro adds to it. This processing of the return variables is part of that core.
If by s2Member Pro you mean the pro-forms that integrate with another service of PayPal that isn't PayPal Standard buttons, then I don't know.
You'd think that it being the same company, a change would apply to the whole service, but each product really is separate. So it's still possible that a change was done to PayPal Standard and not PayPal Pro.
You could enable logging, do a test purchase, and see what the log entries say. Look for mentions of the
custom attribute and whether it was validated or there's an error message.
It'd be interesting to see what your tests show.
Let me know if you do them, and your findings, please.