I think remosito does not play ESO anymore, he said. So I guess noone is maintaining that addon atm.
Let me have a look, maybe it is possible and easily doable.
Edit:
It only seems to work with NPC text that would be shown by vanilla subtitle manager too (ZO_SubtitleManager.OnShowSubtitle), or in the chat as monster say or yell channels!
...
Wait—there’s an add on to stifle NPCs? Does it work on PS5? The guy in Alinor yammering about “unloading boxes” drives me nuts.