Though you could add a chat command, workshop subscription doesn't make it easy.
Additionally, you'd have to update ULX files every single time we released an update, unless you made your own addon folder structure for that specific command.
I personally question WHY one would want a chat command for the steamid.
Most of the information an admin would use to grab a steamid is presented in game console log, and often remains on screen much longer than the chat window. Additionally, chat commands are NOT autocomplete, where as when typing "ulx b" in game console, an autocomplete dropdown occurs, allowing you to quickly type/paste the information mostly automatically.