Zaphod: Tw means "trigger warning"
In other words, 'be warned, sensitive topic'
A rather dubious warning if it isn't understood ? 
Respectfully, I'd argue that it would be understood by most people under the age of, lets say, 40.
Respectfully or not, your (presumably underestimated) age-limit is about half my age. But it may also be true that many RootsChatters are above it anyway, so your 'tw' will possibly not be understood by the majority
But as has been suggested, trigger warnings are a daft idea anyway. If anyone is seriously offended by something which has not been intended to offend, whose is the problem ?