I would still use a 26″ depth for all tall cabinets and appliance case stacks. Even if you don’t have a countertop terminating into the side of the tall end panel (as shown in the previous picture), you’ll still benefit from having the extra couple of inches of space for electrical hookups behind your appliances.
Technically, 24″ depth is fine, and most off-the-shelf cabinet suppliers won’t make anything deeper than 24″ because it causes material-yield issues. The plywood sheets we cut are roughly 48.50″ wide by 96.50″ tall, so you can’t get two 26″ rips out of a single sheet. That means higher material cost for them because they need to cut into an additional sheet and end up with a bunch of unusable strips. We combine multiple orders to maximize every square inch of plywood, so this limitation doesn’t affect us.
Note that 26″ is our standard depth for tall pantry cabinets, but 24″ will work for most applications. Also, for island installations, I recommend making the cabinets 26″ deep instead of 24″. Historically, this extra depth allowed space for things like downdraft vents or additional plumbing runs in the back of the cabinets. It also gives the plumber more room to work when installing a faucet on a sink located in the island.