Third party developers dropping support for OS versions that are, frankly, not even very old, is a scourge in software today.
I can maybe understand sunsetting support if the OS made a huge backwards-incompatible step change, but macOS and iOS updates don't tend to be that kind. The differences (for developers) between Catalina and Mojave are minuscule. Retaining support for Mojave should be close to zero effort on the part of the developer. There should be no difference in maintenance burden between building an application that runs on Mojave and Catalina, and building an application that runs only on Catalina+.
I can maybe understand sunsetting support if the OS made a huge backwards-incompatible step change, but macOS and iOS updates don't tend to be that kind. The differences (for developers) between Catalina and Mojave are minuscule. Retaining support for Mojave should be close to zero effort on the part of the developer. There should be no difference in maintenance burden between building an application that runs on Mojave and Catalina, and building an application that runs only on Catalina+.