If you do want some insurance, the old over-provisioning trick should still work to prolong the life of the drive when used in a system that doesn't support TRIM. Format the drive on a modern system that does support TRIM, or secure erase it with the manufacturers utility, then create a partition which doesn't fill the entire drive and use that on the classic system that doesn't support TRIM. As long as the system never writes to the unpartitioned area it will remain in the TRIMed state and allow the SSD controller to use it as slack space.