"everything is fine until YourLib 2.0 comes out, at which point users doing `brew install myapp` will start getting cryptic compiler errors."

Sorry, this is just not true, we hold merging YourLib 2.0 into core until all dependents are upgraded or boneyarded. Maybe you're speaking from your outdated experience with brew years ago.

Also, we do have plans to support multiple versions of libraries, see https://github.com/Homebrew/brew/issues/620 "Handle Versions Better" (although I'm not necessarily a fan of this personally). We also have an openssl@1.1 formula in core already in case you want a sneak peek of the future.