What you want IINA to do:

Add a menu item Toggle Subtitles to the Subtitles menu. The menu item will have a keyboard shortcut S or ⌥⌘S. Using this menu item will show or hide currently selected subtitles.

What IINA does currently:

IINA currently has a Cycle Subtitles that switches current subtitle to the next option in the first Subtitle submenu. It skips the next subtitle if it's already selected in the Second Subtitle submenu.

Why you think this should be added:

Often in a video that has mostly clear speeches, except for a few seconds of ambiguous or indistinct sound, it's quite a hassle, and immersion-breaking, to go through the Subtitles menu to switch between the desired subtitle and None many times for a total of a few seconds of the video. It's a much friendlier experience to provide the user a way to quickly go back and toggle the subtitle: ← S.

Another use case is for multi-lingual videos, where the user might be familiar with some of the languages spoken within, but not all of them. The user might need subtitles for parts of a video wherein the languages are unfamiliar to him, but prefers not to see subtitles in the rest of the video.

The current Cycle Subtitle menu option covers this feature when there is only 1 subtitle, and if it's not selected in the Second Subtitle submenu. However in situations typical for a Blu-ray release of a blockbuster movie, where there are more than a handful of subtitles available in the video, cycling through them all just to toggle a subtitle on/off isn't ideal.

Examples of other projects that have something similar:
MPlayerX has this feature.