Aye it's ass. Even the temp mail idea is flawed because at some point you need to confirm your identity/ownership of the account.
I did a huge GDPR cleanup of various accounts I owned based on what I had and wanted gone from my password manager at the time. A mix of:
Please fill out this pdf with all your details of all the information you want deleted. If we can't find the data base don that form then it's on you. (Unnecessary amount of work. Solution, report to national GDPR Rep for obfuscation.)
Non-response from any active email. (Illegal for companies with operation sin the EU. Report to national GDPR rep for non0compliance.)
Did not respond for 30 days (Illegal for companies with operation sin the EU. Report to national GDPR rep for non0compliance.)
Asked for an extension to 60 days (Only possible in certain extreme circumstances that they need to prove to you. Report to national GDPR rep.)
Asked for copy of passport to confirm identity. (Unnecessary if emailing from the email they have on file. Tell them this. If they don't delete/ignore report them to national GDPR rep.)
Self-service deletion does not work and customer service will only refer you to that self-service. (Report to national GDPR rep.)
And probably more I don't remember. Maybe 30% of companies I reached out to actually just deleted it and confirmed as such within 30 days. If you're doing similar then you've got to get good at reporting people to strong arm them, especially if they just no respond after 30 days. Gotta proactively stay on top of that.