Any identifier can be a "personal data" only if it allows to link it to some real person. If you don't store IP address, email, real name, ID card number, SSN, bank card number, phone number, then user id is just a number.
Quote [1]:
> ‘personal data’ means any information relating to an identified or identifiable natural person (‘data subject’);
Quote [1]:
> ‘personal data’ means any information relating to an identified or identifiable natural person (‘data subject’);
[1] https://gdpr-info.eu/art-4-gdpr/