The only time ATC should warn you about an aircraft is if it’s possible you’re paths might conflict within some amount of distance, likely 1 mile. If that’s possible, ATC should say something. Given we now have ADSB, that’s especially true, because, especially with airliners, those aircraft should show up on the screen.
I realize that the traffic might be too far away to be seen. There’s lots of time in the air that I’m warned of traffic, say 500 feet below me to my 7 o’clock, I say I’m looking for traffic, I say I don’t see it, and ultimately ATC says it’s no factor.