Which Swift 5.1+ features require runtime support?

When Swift 5.1 was released with iOS 13 / macOS Catalina, there was some discussion of which of its features required dedicated runtime support, restricting them to those new OS versions.

Out of curiosity, I am wondering what features got introduced in Swift 5.2 and 5.3 that require similar new runtime support?

Here is a not yet complete list of things I'm thinking of:

Which features am I missing?

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