Well, even in the 60's there were leads, regulars, semi-regulars and recurring. Ultimately, I don’t think it had as much to do with the credits since that was contractually based and not necessarily how the show developed. Let's take the Irwin Allen shows as an example: Lost in Space and Land of the Giants listed all of their main cast up front in the opening credits. Yet, some of the characters in both shows were marginalized down to semi-regular status. In the case of Lost in Space, the lead was practically a highly paid a day player once Dr. Smith took over.
The characters were marginalized, but they were not
semiregulars. They were billed and contracted as regulars, and they still appeared at least briefly in every single episode, even if it was just to deliver a couple of lines -- with the exception of LotG's Stefan Arngrim, who was absent from five episodes, and Heather Young, who missed seven, mostly due to maternity leave. But since they were
regulars, that means they got credited and paid even for the episodes they weren't in, and got paid just as much regardless of how many lines they had -- which is what distinguishes them from semiregulars or day players, who are credited/paid only for the episodes they do appear in. Acting is a business, so the distinction between job categories is a matter of contract and salary. A regular is someone who gets paid regularly, not who has a consistent amount of screen time.
There were plenty of DS9 episodes where Sisko was not a significant character, but in every case, the writers made sure to give Sisko at least one brief scene with dialogue even if it had nothing to do with the main story. Same with the other billed regulars, other than Jake. Most of the time, even if the episode only focused on a couple of the cast, the other adult regulars would still make brief appearances just long enough to justify their contractual requirements. That's what distinguished the actual regulars from recurring characters like Rom or Garak, who were hired on more of an episode-by-episode basis and didn't get gratuitously written into stories that weren't about them.
The same went for TOS. In season 1, Kelley wasn't a regular, so McCoy was absent completely from two episodes. But after that, he was a regular, so he had to be written into every episode even if it was just to hang out on the bridge and say something snarky. But the other characters were not regulars, so there was no incentive to give them token scenes. They were used when the story called for them, and otherwise were just not there.
However, Voyage to the Bottom of the Sea and The Time Tunnel only listed the two leads. The end credits listed EVERYONE, even the leads. You can still be a "regular" if you missed a couple of episodes. On Voyage, Bob Dowdell (Chip Morton) was indeed a regular, but he missed at least two episodes during the run.
Okay, if an actor appears in every or virtually episode despite not having main-title billing, like Lee Meriwether and Whit Bissell in TTT, then one could argue they were effectively regulars, even if they weren't contracted that way. But how is that relevant to Nichols, Doohan, Takei, and Koenig, who did not
appear in virtually every episode? Nichols and Doohan were in only 80-ish percent of the episodes, the others in considerably fewer. They were semi
regulars at best.