I don’t ever use diet drinks, and my caffeine intake is minimal. Yet I have bad tinnitus.
I’ll occasionally have a few Starbucks coffees per day when on business trips but it does not impact the severity of the tinnitus.
My understanding is that all of the assumed links to tinnitus are guesses based on minimal scientific evidence. Sadly, it will probably be a long time before any real progress is made in the field.