Questions: Since that "modify" variable is now deprecated and optional, why do you still keep it around in the code and not remove it altogether?

I have no clue. We're just following their documentation below.

developer.paypal.com/docs/paypal-payments-standard/integration-guide/Appx-websitestandard-htmlvariables/#deprecated-variables

Specifically "Update your code to use modify=0, the only valid value, which allows your customers to sign up for new subscriptions only.". So we send modify=0 as instructed. Seams someone recently has been tampering with this on PayPals end and has crippled likely thousands of integrations. I assume they're working on reverting that change.

For now if commenting that line out resolves your issue then I recommend leaving it commented out. We still have to wait on official word from PayPal though. I've added a separate bug ticket to track this issue.

forge.joomlapolis.com/issues/8038

In the documentation you are quoting, that filed is marked as "optional" already, so it can be dropped without any issues.

It is now optional. It could not be dropped before without causing new subscriptions to fail so it was left at the recommended modify=0.

Also I just hear from PayPal, quoting their reply here:

Thank you. It's a shame they didn't try to notify developers of this. Will remove the parameter for next build.