In the upper right hand corner of the Memorized Payee List there is a toggle labled "Auto-delete". The gear icon lets you set auto-delete of entries that haven't been used in a specified time period.
I had it set for 13 months and there are entries with "Last Used" dates of 5 or more years old still in the list. Apparently they have to be deleted manually one by one.
Quicken Classic Premier 69.53 release (current) on Windows 11 Pro.