Templates don't have a "last used date" -- as you've noticed.
Additionally, reporting on templates is a little fraught if you have stacked templates (a project where you attach a number of templates, would only ever record the first template attached).
With this in mind, the best way to come at this has been to use text mode in a template report like the following.
EXISTS:a:$$EXISTSMOD=NOTEXISTS
EXISTS:a:$$OBJCODE=PROJ
EXISTS:a:entryDate=2023-01-01T00:00:00:000
EXISTS:a:entryDate_Mod=gte
EXISTS:a:templateID=FIELD:ID
In plain english this translates to: show me all the templates where no project has been created "using the template" after 1/1/23 -- meaning the template probably hasn't been "used" since before 1/1/23. (in this case, "used" meaning "attached as the first template") It doesn't exactly give you a last used date, but it does give you a sense that some templates are evidently not being used.
This wouldn't account for any templates attached AFTER the first template, so you would want to tread carefully here. I've sometimes done this using template task reports looking for the same thing (tasks created from the template task, and if NONE of the tasks in a template were used, the whole template is a good candidate to get deleted)
