Disclaimer: I might overlook some aspects from the Discourse perspective.
significant legitimate interest in preserving the content of conversations
Practically, I do not see any valid interest that “outweighs” the user’s interest in having a clear record if the person wishes so. We would need to put up a balancing test between the constitutional, platform interests and the user’s privacy rights.
This EU document discusses the legitimate interest within GDPR as well (p. 4):
Article 7 [Art 6?] requires that personal data shall only be processed if at least one of six legal grounds
listed in that Article apply. In particular, personal data shall only be processed (a) based on
the data subject’s unambiguous consent2; or if - briefly put3 - processing is necessary for:
(b) performance of a contract with the data subject;
(c) compliance with a legal obligation imposed on the controller;
(d) protection of the vital interests of the data subject;
(e) performance of a task carried out in the public interest; or
(f) legitimate interests pursued by the controller, subject to an additional balancing test against
the data subject’s rights and interests
I assume they discuss Article 6 GDPR, especially Paragraph 1 (might be that six and seven got swapped over time)
Processing shall be lawful only if and to the extent that at least one of the following applies:
(a) the data subject has given consent to the processing of his or her personal data for one or more specific purposes;
(b) processing is necessary for the performance of a contract to which the data subject is party or in order to take steps at the request of the data subject prior to entering into a contract;
(c) processing is necessary for compliance with a legal obligation to which the controller is subject;
(d) processing is necessary in order to protect the vital interests of the data subject or of another natural person;
(e) processing is necessary for the performance of a task carried out in the public interest or in the exercise of official authority vested in the controller;
(f) processing is necessary for the purposes of the legitimate interests pursued by the controller or by a third party, except where such interests are overridden by the interests or fundamental rights and freedoms of the data subject which require protection of personal data, in particular where the data subject is a child.
So lets do some balancing test together. We have the fundamental (privacy) right of the user, it can only be restricted for very special and good reasons or the users’ consent. Then we have the discourse interest to keep the conversations. So practically thinking, if a user posted a picture of them and “deleted” (here pseudo anonymized) their account, they would have no possibility to entirely remove e.g. multiple posted (personal) pictures. Another aspect is very likely that other platforms do not keep conversation data and for most conversations there is no reason to keep old conversations. If there is another method involved to successfully remove private information from the posts and so on, that is automated, I think you can put a balancing test in your favor but from that perspective the user’s interest outweighs the platform’s interest.
“Artistic expression or journalistic expression” (p. 11) does not apply to solely random content on platforms. The authors would need to be (hobby) artists or (hobby) journalists, and it would only apply to individual (journalistic, artistic) posts, where the criteria apply. Same as with public interest (e.g. national security) and freedom of expression (e.g. political or controversial opinion-based posts).
We should also take a look at this (p. 11):
legitimate interests ground, along with the other grounds apart from consent, requires a ‘necessity’ test. This strictly limits the context in which they each can apply. […]
With the best intentions, I can not see as single point that goes into the necessary direction, and just saying the deletion of old posts from account that is being deleted would rip conversations apart (that were barely touched over the years) is probably no valid ground for this. It can be argued that users can just skip deleted posts or do not see them at all, and mostly other users’ indirectly give away the previous posts content, including quotes.
Even more important is the deletion request by the user, that definitively uses the right to object, and removes not only the consent but in most cases even the legitimate interest.
Last but not least, this is the most significant aspect (p. 17):
As the processing of the user’s data is ultimately at his/her discretion, the emphasis is on the validity and the scope of the data subject’s consent.
More generally, currently the user is stripped off the deletion rights in the GDPR that must like previously quoted provide a easy (full) deletion method like you can register easily. Furthermore with the deletion the consent vanishes, and as we could not establish a legitimate interest (yet?), it would be a illegal data processing (no legitimate interest, no consent)