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.
‘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.
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".