I'm not so sure this is a great idea, quite commonly when reverse engineering something I need a good reliable implementation of a dated cipher. MD4 is still used in NTLM, RIPEMD160 is still used in Bitcoin and other cryptocurrencies, etc.

Systems exist outside of the Go ecosystem and not all systems are new. Compatibility is important. It's great to flag these as not recommend for new designs, but it's quite another to remove them entirely and prevent people from easily implementing compatible software. Seems more like something that should be in the documentation or spit a compiler warning if that isn't deemed loud enough.

MD5 is similarly broken but I'm assuming they don't want to remove it because it's still quite popular in some use cases - what exactly is their standard for that popularity?