Are car mods locked behind GTA 6's Ultimate Edition?
Mostly no.
Viral claim: “Car mods are locked behind the $100 edition” Mostly false
Mostly no. The $99.99 Ultimate Edition does add car content: two mod shops called Rideout Customs and One-Eyed Willie's, a Ganado Retro Build and a few exclusive cars.
But nothing Rockstar has published says the $79.99 standard edition can't tune cars. The story that started this was about a handful of in-game stores. By the time it spread, it had turned into "car mods are locked."
We'll check the standard edition's mod shops ourselves after launch and update this page.

Watch us check it
Sources
Until launch this comes from pre-release previews and interviews. We re-check every verdict against the real game after November 19 and note any change here.