In Europe GDPR gives you the right to have all your data deleted. All you do is send in a request and SO has to remove everything of yours, not just anonymize it. There are some exceptions for legal reasons, eg where financial transactions are involved, but comments should not be exempt.
Proton serves privacy, not anonymity. They will not collect, harvest, analyse or sell your data. If you however use their services for illegal things they will forward whatever - usually little - unencrypted information they have about you.