I may be missing something. We have two cars at this household, one a 2021 MY with EAP and USS. It also has RADAR, although my understanding is that that's disabled. The other car is a 2023 M3, bought in September of last year, and had FSD transferred from an older 2018 M3. This car has absolutely no USS.
The 2021 MY has summon; never lost summon; and shows summon on the menus and all that. And it's present on the app, so one can back the car out of the garage and all that jazz. And this is with or without the 30-day trial of FSD. It very definitely has Autopark; we're currently on a longish trip from NJ to Texas. (Eclipses, don't-cha-know). The Autopark on this car doesn't resemble the really fancy autopark visualizations one sees in various release notes. In looks like the old-timey autopark from a few years back.
The 2023 M3 has no Summon buttons anywhere on any menu. The app, when selecting this car, has no Summon section. The car very definitely has the fancy Vision stuff and Autopark. Note that that car is not on any free trial; we bought FSD for a few grand back in the day and, as such, it remains.
So: If you have an older Tesla with USS and didn't have neither EAP nor FSD, you didn't have Summon, because that's not a feature of Standard Autopilot. When you got the FSD free trial, then, what with USS and all, you got the latent Summon that had been hanging around but not enabled.
The 2023 M3 over here doesn't have USS, doesn't have Summon, but, hopefully, will get it one of these days when Tesla releases Actually Smart Summon, which will be running on vision and all.