Since this happened after an interrupted shutdown and not after a Steam update, the Steam client files are likely corrupted rather than the update itself being the cause. On macOS, the steamui.dylib error is often fixed by forcing Steam to rebuild its client files.

Try this:

Completely quit Steam (use Activity Monitor to make sure no Steam processes are still running).
Open Finder → Applications → Steam, right-click Steam, and choose Show Package Contents.

Go to Contents/MacOS/ and run steam.sh if available, or open Terminal and run:

~/Library/Application\ Support/Steam/steam.sh

This can trigger Steam's self-repair/update process.

If that doesn't work, rename the steamapps folder (to keep your installed games safe), then delete the Steam application and reinstall the latest Steam client. Your games can be restored by putting the steamapps folder back afterward.

Since you're on Catalina, keep in mind that newer Steam clients may have limited or no support for older macOS versions, so upgrading to a newer macOS version (if your Mac supports it) is the best long-term fix. If you can, also check the exact steamui.dylib error from the console, as it may point to a permissions issue instead of corruption.