I thought there was something people applied for where you got FSD features early and you were sort of an FSD beta tester? Am I just imagining that? I recall something pretty selective based on driving and you could get kicked out...?
The key thing to remember is FSD is
not FSDbeta.
FSD is a fairly limited set of driver assist programs, basically just Enhanced AutoPilot (parts of which are still beta versions but pretty stable) plus some party tricks (which those newer owners without the Ultra Sonic Sensors don't have and were promised the equivalent party tricks via Teslavision but that has yet to happen.)
FSDbeta was released well over a year ago and originally those who wished to be in the program had to prove they were attentive drivers by spending X amount of time having their driving habits analyzed by tesla. That requirement has ended and the software has progressed to a version that allows anyone who has the appropriate FSD-hardware on their vehicle to subscribe and use FSDbeta without testing to see if they are 'good enough' to be considered a beta tester driver.
FSDbeta is the EAP on steroids that includes the ability to 'safely' use it on city streets but not in any way that could be described as full self driving. You are still required to be driving paying attention to the road and with hands on the wheel. There was a strikeout provision to reprimand drivers who didn't follow the rules. That enforcement eventually added in using the in-car camera to determine driver attentiveness. Now that has been expanded to AP driving as well.
(I see from other posts you have had problems in AP with the latest version of your car recognizing you are meeting those requirements after years of driving in AP successfully.)
Currently (I believe still, as of Jan 2024) it uses a different code than the EAP for highway driving. So if you set your car on AP and drive on the highway, you'll find less robust (but still pretty darn good) behaviour compared to FSDb on the same highway. I have two profiles on my car and can switch back and forth to compare the two on the same road. I don't use FSDbeta on streets but I prefer it to EAP on the highway (but I only use it for lane keeping and TACC, I don't Navigate on Autopilot nor in FSDbeta have the car fully drive so I choose which exits to use, etc, from highways and thus avoid the lane changing nonsense that FSDbeta often exhibits.) For that 'small' subset of features, FSDbeta is smoother and, of course, the visualization on the screen is far more detailed.
The "single stack" that some owners are waiting for is when there is a single version of code for both AP and FSDbeta. While the AP now has the same strike features that FSDbeta drivers face, I don't believe the lane keeping and lane changing code is the same for AP and FSDbeta. If that single stack arrived in the latest update, I will stand corrected. I'm still running older updates while I await tesla to force the latest updates (and updates to those updates) to me over LTE. That hasn't happened yet, so my current version of software is what was released back in November. That didn't take us to 'single stack.'
The single stack I refer to is for the driver assist features only. Those early FSDbeta testers were enrolled into a different update stream and thus didn't get the UI improvements until months after those were made available to non-FSDbeta cars. Those who weren't part of that early FSDbeta stream still had access to FSDbeta when they subscribed, but the version their cars had was an older version than those in the FSDbeta stream.
So for a while there were two streams of cars, those with the latest UI but delayed FSDbeta, and those with the leading edge of FSDbeta but missing UI improvement such as larger font on the screen. With the latest holiday updates, all of us now have the same UI and FSDbeta versions (except for a few older vehicles with AP2 hardware, according to teslafi.) but I do not believe that this version has launched the 'single stack' code for AP and FSDbeta. For driver attentiveness, the two share the same features, but I haven't seen anything that definitive announce they 100% share the same code.