The notion of "limited" is complicated. They are all "limited" in the sense that there are roughly 250 total cars a year, globally. The 06 Heritage had 343 in that paint scheme alone, so in that sense, if every single GT of a particular 2017-2022 model year was the same, it's still significantly less. That doesn't even account for the fact that there will be years like MY21 in which a significant percentage of the model year total are not in North America.
That said, no GT model or version is limited by anything but production capability. You won't find a "limited to XXX units" discussion on anything but the "Studio Collection," which is a paint-scheme only option for 2021 and 2022, in a total of 40 units.
The 2017, 2018, 2019, 2020, and 2021 Heritage are all totals of between 2X units and 5X units.
There are a very small handful of 2017 and 2018 Competition cars, as those cars could not be ordered with air conditioning, and the car is effectively unusable without AC other than being a showpiece.
The Standard car and 2019+ Carbon Series are produced in similar quantities. The Carbon Series is a full package that includes a model-specific interior.
The full bare carbon body car starts at ~$775k, and again, it is not "limited to X units." There will be around 30 of them globally from late 2020 until the end of production.
You are unlikely to notice a performance difference between 2017-2019 and 2020+ unless you spend a lot of time on a road course. The 2020+ cars have a superior calibration and cooling for the track. The HP is negligible.
The last of the 1,350 cars will be delivered in the early portion of 2023.
I think there will always be some elevated level of demand for Heritage cars. Other than that, I don't think it particularly matters much, especially if you are looking to buy one to use as an automobile vs. a speculative investment.