Kexts are not deprecated in general-- only kexts that use deprecated KPIs are deprecated. (The page you link is the list of deprecated KPIs.)

The net effect of this: if something can be done using a System Extension rather than a kernel extension, you'll get deprecation warnings if you try to do it with a kernel extension. Kernel extension points that have not been replaced yet are still valid, will still be signed if used, and will still run on current versions of macOS.