I can't speak to GDPR specifically because I'm not European, but a fair amount of laws have leeway for preservation purposes. (For example, section 108 of the Copyright Act in the US functionally exempts archives from being punished for copying provided they are doing so for preservation purposes).
There are very good reasons that archives will not destroy or alter information outside of very clear difficult and manual processes.
And actually, looking at it, I don't think they're necessarily in violation of GDPR [0].
Point 3 says: "Where personal data are processed for archiving purposes in the public interest, Union or Member State law may provide for derogations from the rights referred to in Articles 15, 16, 18, 19, 20 and 21 subject to the conditions and safeguards referred to in paragraph 1 of this Article in so far as such rights are likely to render impossible or seriously impair the achievement of the specific purposes, and such derogations are necessary for the fulfilment of those purposes."
According to GDPR, national law of EU parties overrules GDPR when it comes to personal data being used in archival context. I don't know every EU country's stance, but most of the bigger economies would allow for this.
There is also a difference between deleting the data and rendering it inaccessible to the public. Keeping something under wraps is generally more 'acceptable', but active destruction of the item (digital or not) and its providence is much more limited. Also there's a difference between personally identifying data (covered by GDPR), your content (which would be covered under copyright and not GDPR), and connections people can make if that content is available (not covered at all because it's not anybody else's issue if you write something terrible and people keep recognizing you over it so long as you did actually write it).
[0] https://gdpr-info.eu/art-89-gdpr/