This was deliberate, and here are my reasons....

1. Minimal Impact.

The desired effect seems to be to cause the minimalist interruption possible, it doesn't break existing macros. This means the list of icons is still there, but you just can't set them anymore.

/macroicon only works for learned abilities, but 95% of the time, someone is using this for the clock feature (showing timing of the ability) anyway. i.e. Why would you use it for abilities you don't have if you don't have the ability? I have already checked, my archer is level 9, I can set /macroicon "Heavy Shot" and I get it, but if I use /macroicon "Swiftsong" it doesn't work. Otherwise, you'd use a static icon... (now from a limited set) ... I have more to say.....