The core of optimization in almost any version of D&D has been "tap the enemy more".I don’t think a spell that just gave a situational reaction attack would even be worth a 1st level slot.
If you are in melee range with a spellcaster, they have to either (a) disengage, (b) suffer a reaction attack to move away, or (c) cast a spell.
Disengage usually requires burning their action. This ability closes down the (c) option. So you are upping your "tap count" by 1, or shutting an enemy down for a turn.
That is an insane effect for a 1st level spell slot, especially past low levels. I mean, 1st level slots for smite (9=2d8 damage) is considered a great use; a tap could easily hit or even exceed 2d6+18 (25) damage (20 stat, +3 item, +10/-5 feat, 2d6 weapon).
It does require using a spell known/prepared, but using the slot on it once you had it prepared would be obvious.