So as of Patch 5, stealthing with impunity outside of an enemy's vision field is still a thing. The problem with this is that it doesn't take sound into account. You can be wearing full plate and moving behind an enemy all day and as long as they or nobody else sees you, you can Hide to your heart's content (unfortunately still as a bonus action; hope that gets the Disengage treatment next) and get all the advantage you want in melee.

I hate to keep bringing up Solasta (well, no, I don't, but you know), but the pulsing circles around characters when they stealth in that game would be very effective for representing the sound element of stealth and detection. The higher your Stealth score, the smaller your circle is, just like Solasta, representing your ability to move silently. And if you move near an enemy and your circle overlaps them, then the enemy should get to make a Perception check, even if you're not within their field of sight.