It depends on the lounge!
Sometimes.
If you are flying international you are often filtered out through passport control/customs, so you can only access an official “arrival lounge.”
In the U.S. flying domestically, I usually can get into Priority Pass lounges because a “same day boarding pass” grants access.
Also of note, Capital One Lounges for example explicitly say you can’t visit on arrival, you need an outgoing boarding pass, but the scanner doesn’t flag anything in DEN at least.
A friend and I have both had no issue going upon landing.
I assume an employee would need to catch it/no one actually cares, or it’s simply a loophole they’ll eventually close.