All RT request from individuals where the requestor, owners, or cc:s can be identified by f.x name or email adress or the context of the request, are Personal Data (PI) per the definition in the GDPR.
I agree that RT system owners can use legitimate interest as an argument to not require initial consent, and for the continued use of personal information for the duration while an issue is active, and if relevant also for a limited period after that for say billing, usage analysis, or process improvement.
But the GDPR also clearly require that the processor does not require or collect more PI than necessary . The GDPR also requires that collected PI isn’t stored longer than necessary.
Because of that I do not agree that one can blindly store all requests in an RT system indefinitely and claim that it falls within the scope of legitimate interest. It is reasonable in general that old requests are deleted when no longer absolutely relevant to the business relationship with the persons involved in the request.
Perhaps RT already has a date based auto-purge option? Otherwise it would be neat to have. Basically that RT by default purged all non-active (resolved/rejected/deleted?) requests after say 3 month. That would make GDPR compliance with RT a breeze.
 data minimisation, Chapter 2, Article 5.1 ©
 storage limitation, Chapter 2, Article 5.1 (e)