Disfavor another `unimplemented` deprecation #407

Job	Run time
Examples (Debug)	7m 20s
Linux (debug)	45s
Library evolution	26s
Examples (Release)	5m 39s
macOS (debug, 15.4)	48s
Linux (release)	44s
macOS (debug, 15.2)	53s
SwiftWasm (wasm-5.9.2-RELEASE)	1m 3s
macOS (release, 15.4)	38s
Windows (windows-latest, debug)	1m 26s
Windows (windows-latest, release)	1m 52s
SwiftWasm (wasm-5.10.0-RELEASE)	48s
macOS (release, 15.2)	44s
	23m 6s

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