What you want IINA to do:
Download from Open Subtitles using their new REST API.

What IINA does currently:
Downloads from Open Subtitles using their original XMLRPC API.

Why you think this should be added:
Because of this post by Open Subtitles: OpenSubtitles.org API deprecation and end of life which says:

After 16 years of service, the time has come to turn off our original API service, to focus our time and energy on the new REST API.

And:

2023-12-31 end of life, API endpoint will be turned off

IINA has no choice but to switch to the new REST API before that deadlline.

Examples of other projects that have something similar:
N/A