Yes, Shield Master is a feat. It also offers two additional very good features when you have it so of course it has a higher "cost".
Shield master imposes the prone condition until the target's turn, allowing allies to benefit from the target being prone as well as the attacker for tables who allow the bonus action to precede the attack action (which many do IME). It allows OA by the prone target, even if at disadvantage. For a build around Shield Master, that already blocks other build types, so... not seeing your point there.
There are 4 melee weapon wielding layouts. DW, S+B, GW and Single weapon.
There are 2 melee stats -- Str and Dex.
Shield Master restricts you to investing in Str (to get the shove), and to S+B melee. This locks you out of high-damage GWM and SS feats. These are significant costs.
This cantrip does none of that.
I can use this on a GWM build. I can use this on a pure caster to grant advantage on my spell attacks. I can use this on a SS build. Whole piles of things you can use this for that wouldn't be possible for shield master.
I mean, shield master can be used with spell attacks, but now your character has to invest in strength and athletics and a feat and spell attack stat, and be adjacent to the foe: these are huge costs. With this? It costs you a utility cantrip and your bonus action for a plausible 30%+ increase in damage.
On almost every single melee build if given a chance to poach this, I would. On almost any caster? I'd want it. There is almost no downside to this cantrip, the price is dirt cheap, the payoff pretty good.
The revised cantrip (with my posted comment, which I guess you ddin't read?) would also target only 1 creature. It also only allows you to benefit from it since it ends at the end of your turn. It requires one of your limited cantrip choices (most classes only get 4-6).
There are many versions flying around. The one I was talking about was the bonus action, blind only on the turn, higher levels more targets.
The more targets was just a cherry on top of an OP Sunday.
And so what if allows you to try to escape or hide? The target would still get an OA even if with disadvantage (so it doesn't nullify OAs as you seem to think).
"You can make an opportunity
Attack when a
Hostile creature that you can see moves out of your reach "
Blind cannot see. Cannot made OA. So it acts as a disengage action (if it hits) as well.
In comparing it to Warding Flare [...]
Warding Flare is a class feature, not a cantrip. Warding flare doesn't impose blindness. Warding flare doesn't grant advantage. Comparing this to warding flare is pointless, because the two have nearly nothing in common, other than not working on targets immune to blindness.
As others have said, my overall big concern is this would be a way to give yourself advantage on attacks or spells cast afterwards.
So
your overall big concern is ...
the main thing the cantrip does. Wow.