Eg a typical use case, a value is set to something and so you look at the config log to see when it changed but it never did, or it was years ago. It might have been set to A in the db, but then forced to B a year later in config.php and you just can't tell when. In theory if you change config.php then that should be under some sort of version control outside of Moodle so you can argue this isn't something moodle should attempt to solve.

In many cases the forced config items are ones which you don't want to expose in the admin UI so I would limit this to only those which have an admin setting.

It may also be quite tricky to detect when a forced setting has changed, we would have to track the state of whats in the db and what has been forced and if the forced value is changed then we update the state, log it. It's not clear if we'd want to do this during the boostrap or only when visiting the admin setting. Both of which would give a misleading timestamp of when it changed but the latter of would be worse.