[Pitch] [Meta] Call out backwards-deployability in Evolution proposals

[Douglas_Gregor]

@Jumhyn

Perhaps a better way to frame this issue is "Runtime and Standard Library surface impact"? As @xwu notes, "backwards deployable" is (today) an Apple-specific concern over which the Swift project has no purview. But if a proposal indicates that it requires additional runtime entry points or standard library changes (which cannot be emitted into the client), that's a pretty strong signal that any Swift-in-the-OS platforms (currently, Apple platforms) would need additional, perhaps significant, work to support the feature in a backwards-compatible manner.

The Core Team discussed the issue, and their view aligns closely with the above: backward deployment is a vendor-specific issue, and not part of the evolution process. However, calling out the runtime impact of a proposal can help us understand concerns around back-deployment and also the overall impact of a feature on the language and runtime, which is valuable.

The global actors proposal under review has some interesting runtime impact, so I've sketched a short section to talk about its runtime impact as a sample: [SE-0316] Address feedback from the second review by DougGregor · Pull Request #1400 · apple/swift-evolution · GitHub

Doug