The release of Studio 2021 will require all existing plugins to be updated. Plugins suitable for Studio 2019 and earlier will not work in Studio 2021. The reasons are documented in this wiki. So, all submissions of Studio 2019 plugins need to have the minversion AND the maxversion attributes set in the plugin manifest as follows:
<RequiredProduct name="SDLTradosStudio" minversion="15.0" maxversion="16.0" />
Note the use of maxversion="16.0" which is the appropriate version number for Studio 2021.
If you are in doubt we will accept 15.99 where 99 refers to the CU version which would never likely reach 99.
If plugins for 2019 are submitted for publication on the SDL AppStore without this version control in place they will not be accepted.