Any unique identifier assigned to an "identifiable" person is PII. It doesn't matter if it is an IRC or Instagram handler, or a twitter username. It is PII if it can be associated to a person.

Euro/GDPR: https://gdpr-info.eu/art-4-gdpr/

‘personal data’ means any information relating to an identified or identifiable natural person (‘data subject’); an identifiable natural person is one who can be identified, directly or indirectly.

The keyword here is indirectly.

Australia: https://www.legislation.gov.au/Details/C2019C00025

personal information means information or an opinion about an identified individual, or an individual who is reasonably identifiable.

(a) whether the information or opinion is true or not; and (b) whether the information or opinion is recorded in a material form or not.

The keyword here "reasonably identifiable".