I use Airportr to handle the suitcases, so with a boarding card you just go straight through security and don't go near a check-in desk. I travelled recently with a trolley bag, a smaller bag and an A0 poster tube in Club World, just boarded when called and nobody said a word. This was in April 2019. I figured that once you get to the gate, if someone is unhappy with you carrying a poster tube then they will probably just take it off you and put it in the hold, worst case.

I did check a poster tube in once on a flight to Miami. When I got to Miami US customs had done an inspection, taken the poster out and just stuffed it back into the tube, completely ruining the poster. I didn't realise until I got to my final destination in another country so nothing could be done. Certainly wouldn't risk checking a poster tube in again.