There is no exploit or exploit-like mechanics involved here. It might just be annoying griefing, but it's using intended game mechanics.
Though I'd imagine in d2r it's even worse because your character loads in before your load screen, when d2 was instant. If you want behavior like this to stop you'll have to change how the game plays completely and ActiVision Blizzard isn't going to do all of that.
Using intended mechanics doesn’t automatically make something legitimate gameplay — intent matters. Deliberately stacking mobs on a portal to force unavoidable deaths isn’t ‘hardcore risk,’ it’s engineered griefing. Hardcore is about managing danger you can react to, not dying during a load-in because someone set a trap you literally can’t counter.
And saying ‘Blizzard won’t fix it’ isn’t a defense — it just admits the behavior exists and relies on a technical limitation. Reporting it is exactly what players are supposed to do when mechanics are abused to bypass fair play