It’s dependent on the house-rules. But my guess is that most casinos won’t allow it. Also, this might also be a situational question.
It might make a huge difference if you show the card(s) in a multiway pot when you have people behind you to act, or showing it to your opponent heads up in order to get a read while making a river-bet decision (call/fold). In Multiway situations with people behind to act it’s definitely considered bad etiquette, since this could influence players to make moves against others that they wouldn’t make without that information.
Here’s a video from Doug Polk explaining this more in detail based on a hand played at the WSOP 2018. In this video, the cards aren’t even shown. Sometimes, talking is just as bad etiquette as showing the holecards.
Youtube - Why Everyone Is Freaking Out About this Phil Hellmuth Incident (2018 WSOP)