Users need to have at least 12.1 to find and install any paclet from the Wolfram Language Paclet Repository.
It's up to the developer to determine which versions are supported by the content of their paclet, and there is a field in the paclet resource definition notebook (and the PacletInfo.wl file) where you can specify that version. I'd encourage you to build a paclet using the "Build" button and then install and test it in the minimal version that you claim to support. For a complex package, it's not easy to deduce all the dependencies without testing.
