why does everyone say this

Against the rules to trade with outside currency and people have 'said' they have been banned for mentioning jsp.

GGG considers trading items for forum gold as if it's RMT.

It's just people being cautious.
There is no actual evidence of people being banned for doing that, but you don't need to talk about jsp in game so might as well leave it out.

I've seen a couple of topics where people claimed to be banned for trading here but it's all very vague.

Someone just needs to take the plunge and purposely try to get themselves banned / flagged to test

I have mention "fg" "jsp" several times in game by accident. Haven't get banned. I don't think they have somekind of spy program which pick up certain words and ban ppl because of that. I think that u can get banned by using jsp if somebody report u with evidence to GGG by email or something.

You WILL get banned for mentioning ANY part of this site on PoE.

I have had jsp in one of my char names for 2 years.
I have mentioned fg half a dozen times, hell once i intentionally talked about jsp to whiskers just to mess with him.
0 bans.

I mention it all the time, my account is the exact same as my jsp name, watch me not get banned.

proof?