The Fee plan has features and limits, which include an access limit on proofs. Proofs are restricted to a 60-day visibility period. After this period, proofs older than 60 days are "disabled" and will not be editable in the Dashboard, nor viewable in the Proof Viewer.
Please note that the 60-day period for proof access begins from the initial creation date of the first proof version. Adding subsequent versions does not extend this period; the entire proof expires 60 days after the V1 creation date.
This change has been announced with the 24.03 sprint release.