What you want IINA to do:
This is a request for IINA to directly support the mpv options sub-visibility and secondary-sub-visibility. These options disable/enable the rendering of subtitles, similar to muting audio.

Support must include menu items to toggle the state. The menu items must adhere to Apple's Human Interface Guidelines on Menus, specifically the section on Toggled items.

IINA already has a Subtitles menu so new menu items must be under that menu. The current convention in the menu is to use the prefix Second for items that pertain to secondary subtitles. I suggest using Visible and Second Visible for the names of the items with checkmarks to indicate the state.

The menu items must include associated keyboard shortcuts. The key bindings used by mpv are:

IINA does not currently contain bindings for these keys so IINA should use the same bindings as mpv.

When subtitle visibility changes an OSD message must be shown so the user knows if subtitle rendering is enabled or disabled.

This request originates out of discussions concerning issue #4234.

What IINA does currently:
Currently this mpv feature is only available through custom key bindings, which does not provide feedback to the user as to the current state of subtitle visibility.

Why you think this should be added:
Subtitles is one area where IINA has a feature not found in mpv (without plugins), the ability to download from web sites that provide subtitles. IINA should insure other subtitle functions are supported. The mpv subtitle visibility feature is a convenience feature similar to being able to quickly mute audio.

Examples of other projects that have something similar:
This is a mpv feature.