So I did not realize this at first (would have helped if I were, you know, emailed some information about this), but my project was still using a free Postgres addon despite being on a paid dyno setup. So that I suppose was nuked along with the general free tier deprecation.
"I discussed this with our backend team and upon deeper investigation, we were able to confirm that there was some issue sending the notifications to your email."
As I said, unlikely. But not impossible. My work has a GSuite setup and we've had a few verifiable bounces over the years, though they weren't google's fault but were the fault of glitches at our DNS provider (we don't use google for DNS).