7.3 adds support for specifying plugin requirements via Sparkle - realmacsoftware/RWPluginKit GitHub Wiki

1. When requesting the appcast feed from the developer's server, we'll append rw_version, rw_build and addon_version parameters. These could contain the version (7.3) and the build (18546b), which can be used to decide which updates to offer.

For example: http://yourserver.com/plugin/appcast.xml?rw_version=7.3&rw_build=18547b&addon_version=3.2.6&addon_build=3904&macos_version=10.12.3

If these parameters are not set, you should assume that the user is on < 7.3, and should parse the User Agent header instead:

RapidWeaver%207/18546b CFNetwork/807.2.14 Darwin/16.4.0 (x86_64)

Split by space and then by /, and they'll know RapidWeaver 7 and 18546b. Not ideal, but good enough.