I wouldn't chance it; the first time I implemented IPNs, I had to spend almost a week on R&D aimed purely at discovering and working around all the undocumented fuckups.

Webhooks are not hard. That Paypal fails at them this badly makes me think their infrastructure is a (much larger) disaster waiting to happen; if I still did freelance work, I would be moving all of my clients to Stripe right now, before the next inevitable mishap.

They actually do the right thing with a 301 redirect, and re-post to the new URL. I did a site wide http->https redirect, and surprisingly the IPN script continued to work.