I believe compression algorithms don't belong in the core stdlib (i.e., the default "Swift" module that's imported by every Swift program); they should rather be loaded with an explicit import statement.
On its platforms, Apple provides the Compression framework, which provides a unified interface for OS-provided compression algorithms. This framework is accessible to all Swift programs running on these platforms. It's not available on other platforms and the API it provides is outside the scope of the Swift Evolution process.
It would probably not be a good idea to reimplement the existing compression libraries in pure Swift. The existing code is already highly reliable, universally portable and well-optimized for popular target platforms.
However, it would be nice indeed if we had uniform APIs for compression that are usable on all supported platforms. This strikes me as a project that can and should be developed as an external SPM package, then perhaps pitched here for eventual inclusion. (Yes, we may need to come up with new processes to support this -- however, in the meantime, it shouldn't be too hard to import an external package.)
Some elements may lead to pitches for potential stdlib enhancements -- for example, it may be interesting to discuss if the APIs for streaming compression can/should be generalized to a stdlib construct, or if the existing stdlib facilities for dealing with contiguous byte buffers are adequate.