What you want IINA to do:

Monitor FSEvents changes to auto-reload subtitles if they are modified.

What IINA does currently:

You have to manually drag the modified subtitle file to the IINA playback window. Dragging onto IINA icon on the dock doesn't work.

Why you think this should be added:

Helping subtitle makers debug their subtitles. Some famous AAS subtitle tools like Aegisub are not updated for years and have accuracy problems in playing videos inside themselves, hence the necessary of using a separated video player to debug the subtitles. In this case, IINA is almost the best except having lack of the feature requested in this ticket.

Examples of other projects that have something similar:
vChewing Input Method (reloading userPhrases) https://github.com/vChewing/vChewing-macOS/tree/main/Packages/DanielGalasko_FolderMonitor
Using the following solution:
// (c) 2018 Daniel Galasko
// Ref: https://medium.com/over-engineering/monitoring-a-folder-for-changes-in-ios-dc3f8614f902