Another "note to self" post.
PayPal have changed things so that you need a business account to set up an "IPN" notification endpoint. In their own words:
Instant Payment Notification (IPN) is a PayPal feature that sends messages about payments (and other transactional events) directly from PayPal to your website(s)' back-end systems.
My PayPal account once used IPN, but no longer needs to. The IPN destination URL is no longer active, so I was getting emails from PayPal regularly to point out the IPNs wouldn't work, and asking me to change the URL or disable IPN on my account. But because my account was not a business one, I couldn't make the change.
It turns out this is slightly similar to a problem I had enabling a security key for 2-factor authentication; the new PayPal website doesn't contain the page you used to use on their old website to set this up. The solution there was to find, online, what the old URL was, and it turns out you can still use it.
So here. Log in to your PayPal account, then visit https://www.paypal.com/cgi-bin/customerprofileweb?cmd=_profile-ipn-notify. You can amend your IPN destination, or disable IPN altogether.
I'm sure, at some point, PayPal will turn that old page off, but for the time being it leaves a way to access these settings.