What you want IINA to do:
Display in OSD (in the upper left corner in the similar manner how an audio track data is displayed when the used cycles through audio tracks for example) some kind of feedback when a custom mpv command is executed. In my case I have a key mapped to cycle sub-visibility. Ideally the OSD should show if sub-visibility was enabled or disabled in such a case.

What IINA does currently:
No feedback in the UI so I am not sure whether I enabled or disabled sub-visibility (in cases when subtitles are not show at that exact moment in the video).

Why you think this should be added:
Improves the UX. Hopefully that feature could be added relatively easily and it would cover large amount of use cases (many custom commands set by users).

Examples of other projects that have something similar: