Apparently the word "niggling" is forbidden.

If you were to care to read a dictionary you'd find it's definition is anything but offensive:

niggling present participle of nig·gle (Verb)
Verb
1.Cause slight but persistent annoyance, discomfort, or anxiety: "a nasty leg wound which still niggled at him".
2.Find fault with (someone) in a petty way: "he loved to niggle and criticize people".

Could this word be unbanned please?

Chat filters on TF2 servers are imperfect. Niggling is being stopped by a filter that deals with the word nigger and variations. You'll have to use your vocabulary and work around it.

Sorry for the specific problem, just wanted to make a point about how the word filter would benefit from an update.

We are rather niggardly when it comes to updating the word filter.

I don't believe that one, trifling, instance in 3+ years is significant justification of rewriting/updating something, at least when it comes to me devoting time to it. If anyone else would like to add a white-list function to the filter script, then do let us know.