It might not be the "in thing", to train the abs directly, but like I said here, there are four basic functions of the abs (flexion, rotation, anti-flexion, and anti-rotation) and there should be no problem for most people to address them all through the week.
If the schedule is tight for whatever reason, just focusing on anti-rotation and anti-flexion exercises translates to being stronger and healthier, while still stimulating some growth in that area. To prioritize appearance, if "having great abs" is someone's exact goal, you still get more bang for the buck focusing on fat loss (diet and cardio), on top of some direct ab work.
It's probably debatable, but trying to build ab muscle without intentionally reducing bodyfat runs the risk of building a blockier midsection. The kind of abs that might feel solid to the touch but still look like a sack of potatoes when you've got a shirt on.
Same reason training wrist flexors isn't talked about much. The abs do get lots of "accidental" work, but there's nothing wrong with training them directly as long as you do it smart (proper exercise choice, volume, and intensity). If you really want to have them standout, then sure, isolate a bit more and crack down on the diet.
"6-pack abs" technically translates to having low enough bodyfat to reveal the linea alba and the tendinous inscriptions across the rectus muscle. Since their shape is largely genetic, it's why some people won't ever get more than 4-packs, some have wobbly/funky diagonal 6-packs, and some have freaky 8-packs. Even Arnold only had a 4-pack, at best. But that's not about diet or training at that point. It's nature.
Increasing the size of the rectus won't carryover to seeing a "better or deeper" 6-pack if the person's still a fatso.