Hello,
Firstly, we need to advise that your release of SDM (12.9) will soon be out of support and we recommend that you consider migration to one of our later releases of SDM (14.1 and 17 are currently available)
On examination of the question itself, one thought to consider may be to use the auto-close function, where a user marks a ticket as Resolved, and after 60 days, the auto-close then closes out the ticket (sets status from Resolved to Closed) and from there, use the option "edit_inactive" to prevent the Closed ticket from being reopened. The only thing to keep in mind is that a Resolved ticket remains "active" and can then be edited and acted upon like any other active ticket.
The following suggestion is unsupported and we will not be able to assist in any further development of this concept:
If you are using the "Close" status and wish to maintain some functionality of allowing end users to reopen a ticket with status of "Closed", you could also create a script that would run on a periodic basis and sets a hypothetical customised field that flags if a ticket is over 60 days. Script would run a pdm_extract of all tickets with the close date set to a certain value, then modify the extracted entries to turn on the hypothetical field, then use pdm_load to return the ticket entries. You would then use a DP to control pre-updates based on the value of this hypothetical custom field.