I think the logic in 3.5, and I assume the others, for requiring discrete actions for certain uses of social and other skills was based on mechanical balance. Intimidate as a combat action actually contributed a mechanical effect, as did Bluff and other social skills. You could demoralize an opponent and cause them to have the Shakened condition with Intimidate or feint in combat using Bluff and cause them to lose their DEX bonus to AC. If you could do that for free each round, it's just a little OP, without the investment of feats or talents.
I don't believe the same is true in 5E. Instead, it's entirely up to the GM's adjudication on what can be possible through skill use (particularly social skills) and what effects that might have during combat. I'm not sure that is a reasonable trade-off there, unless the GM has previously set forth what can be accomplished through skill use. I actually kind of prefer not having much in the way of in-combat mechanical benefits attached as you end up with Diplomancer type characters.
With that said, if you're attacking and trying to intimidate at the same time, I don't know if there even needs to be a roll. An enemy's morale is going to be largely dependent on whether your battle skills are backing up your trash talking. Alternately, one could flavor an intimidate or persuasion action using alternate descriptions, such as shooting the ground at the enemy's feet like the Gand did or pulling out a medpak to try to get folks to stand down in order to care for the injured like Cal did.
