What a lot of people do in deathmatch is completely and utterly useless to them:
every time they see an enemy they default to panic crouching and spraying in the body.

And since they get some kills like this, they think it's the way to go - which makes them way worse than they should be.

HSDM would benefit lower elo players:

Adding HSDM would improve people's mechanical skill, both movement and aim, since they would get punished for crouching + spraying every single time they do it. It would also improve crosshair placement of lower elo players, because if they go into a deathmatch and look chest / leg level, they will not get a single kill and they would realize they need to change something up.

HSDM would benefit higher elo players:

Other side of the spectrum is higher elo players. When they queue for a deathmatch, they're not there to shoot sitting ducks who do nothing but crouch in front of them. Not only do they not improve from that, the people that are doing it are also just wasting their time.

Preferably HSDM should come with all the other things people are asking for when talking about deathmatch:
-Way faster / instant respawn
-Get heal + full ammo when you get a kill
-Possibly more people in a lobby (i personally don't think that's necessary, i've just seen people ask for it)

I don't see a reason for there not to be a HSDM option in Valorant.
If you ask any CSGO player who wants to improve, not a single one would say they queue for official deathmatch, every single one of them will say they go on a HSDM server / something similiar, and there are obvious reasons for that (mainly the ones that i listed).