I've set up a Swift 5.3 branch on Swift Numerics that adds some extensions for the new Float16 type with 9999 availability (specifically conformance to Real, which also makes Complex<Float16> available). This branch is only compatible with the Swift nightly toolchains.
Sometime after Swift 5.3 ships, I will merge this to master. In the meantime, any Float16 work will go only onto the 5.3 branch, and any other work will be merged to both.