So far as I know, the A/B experiments haven't happened in "RTM" (or "RTW" as more accurate) builds, only Insider builds (which are opt-in). The Insider blogs have also been generally transparent in which tests were performed, at least after the fact, but big ones were also announced well in advance to avoid confusing users.
The biggest/wildest example that was A/B tested to date was Sets. Sets was a feature/system where nearly every window in Windows was heterogeneous tab-capable. (Illustrative example: Open a Word document, add a tab for some OneNote notes, add a browser tab for some research. Save it all as a Set to the Windows Timeline as Set, open it all back up at once.) But ultimately Sets never made the cut out of testing (too many compatibility issues, not enough telemetry usage, too confusing a UX sometimes, not enough apps in the wild with Windows Timeline support, etc) and it sounds like the project is now dead.
(Supposedly it was partly dropped too because apparently they were piggy backing on Edge's UWP tab control and with Edge going "Edgmium" that tab control was considered less critical infrastructure to any team and less of an "in-box" control to piggy back off of. Microsoft Terminal got a version of the UWP tab control pushed out into the open source UWP control toolkit, but probably not in time to be reconsidered as an in-box control for Windows.)
(I was sad I never ended up in the group with Sets turned on. I wanted to play with it.)
(ETA: Also, I really hope as Edgmium gets closer to launch it uses a XAML Island to pick back up the UWP tab control where possible. That would be one great way to feel a bit more Edge and a bit less Chrome knock-off.)