There are two different factors here: The hardware and the software that runs on it.
The 1900/2000 are listed as "Active" meaning they don't currently have a defined End of Support/Life date.
Anything that is listed as Successor Product Available is expected to run a future version of R82.00.xx.
To address the issue of "R81.10.xx on the 1500 / 1600 / 1800", these appliances have limited RAM compared to a 1900 / 2000.
We ran into similar issues with an earlier generation of what are now Quantum Spark appliances, which is why some appliances were capped on the version they could run, even though there were later R77.20.xx releases available.
For specific examples, see: https://community.checkpoint.com/t5/SMB-Gateways-Spark/Latest-firmware-builds-for-77-20-xx-SMB-appli...
Which means that, until there's a notice to the contrary, you can assume that R81.10.xx will be actively supported on 1900 / 2000 appliances.
Having said that, I would not expect many more features to be added to R81.10.xx releases.
I also expect the End of Support date for R81.10.xx won't be announced until after R82.00.xx is available for the relevant platforms.
If you need more clarification on any of this, I recommend reaching out to your local Check Point office.