Hi.
You can continue to use PB 2019 R2, but you can't expect to get support for that version. About your questions:
Continue with PB 2019 R2 until the migration to PB 2022 R3 is complete (approx. 12-18 months). What are the repercussions of continuing to run on PB 2019 R2 Build 2353 & PS 2020 Build 2323 (for web builds)?
You won't be able to get support. In my opinion this is a serious problem.
If we migrate to PB 2019 R3 and do web builds, will the web application still run on PS 2020 Build (2323)?
You should use the appropriate version of powerserver. Last release of PB 2019 R3 is 2803 which also provides PowerServer. Be aware that previous versions of PowerServer are already EOL... To be able to get support for PowerServer 2020 you should be running on PowerServer 2020 build 2323, 2703, 2758, 2779 or 2803 (or to the equivallent PowerServer for PB).
Is PB 2019 R3 version and PB 2019 R2 Build 2353 version similar in architecture? Will web end users find it a smooth transition when we upgrade to PB 2019 R3 web builds? (example, IWARunner, downloaded files etc.)
There was a big change in PB 2019 R3, as in this version the IDE was separated from the runtime. This affects the way native application should be deployed. You can read the following chapter from documentation: Decoupling Runtime and IDE - - What's New (appeon.com). PowerClient deployment was introduced (PowerClient deployment - - What's New (appeon.com)). Those changes were the most important in my humble opinion. There are some improvements related to PowerServer (Improvements - - Release Bulletin for PowerBuilder (appeon.com), Improvements - - Release Bulletin for PowerBuilder (appeon.com)).
For other new features you can check New Features - - What's New (appeon.com).
If we have to move to PB 2019 R3, which build should we move to that is compatible with PS 2020 Build 2353?
I guess this should be one of 2779 or 2803. The main change in those is that the later one uses a different webbrowser control. So it depends on what your application is doing... Both are pretty stable.
Andreas.