The point is not that it doesn’t collect personal data, the _t cookie identifies you as a person as it’s directly tied to your user account’s session on the Discourse server. So having the authentication cookie set in your browser means you’re identifiable for the server whenever you use your browser to visit the website that set it (and so you’re not anonymous). From the European Commision’s Data Protection Working Party WP 29 Opinion 04/2012 on Cookie Consent Exemption - 00879/12/EN WP 194:
Persistent login cookies which store an authentication token across browser sessions are not exempted under CRITERION B. This is an important distinction because the user may not be immediately aware of the fact that closing the browser will not clear their authentication settings. They may return to the website under the assumption that they are anonymous whilst in fact they are still logged in to the service. The commonly seen method of using a checkbox and a simple information note such as “remember me (uses cookies)” next to the submit form would be an appropriate means of gaining consent therefore negating the need to apply an exemption in this case.
Note that they suggest a simple solution websites can implement, a “Remember me” checkbox in the login form, which of course is available almost everywhere these days (but not in Discourse). This was a decision from 2012, so before the GDPR came into effect, but I don’t see any information that it has been superseded (e.g. Cookie Consent Exemptions: Strictly Necessary Cookies - CookieYes updated August 2023 still mentions persistent authentication cookies need consent). But there might be some legal followup or developments I’m not aware of.