"Wtf" word.

It's not like you are gonna be using it to insult someone and you can't tell tones from the chat message so wtf by itself is never gonna get you banned

Depends on the context.

"WTF?! That damage was insane!" "WTF? I thought I hit smite!"

These are fine.

"WTF are you THINKING? Why weren't you with us?" "WTF, dude. Play with the team."

Those aren't as fine, as they're used to emphasize the perceived stupidity of another player's actions.

no

Support told me it's bannable word, coz it means "what the FUCK" so... Confused now.

Yes, obviously it should be reported and punished by permanent ban. How dare anybody say WTF? :O

Cursing isn’t against the rules. Cursing AT somebody is against the rules.