Can I safely assume these kitchen soffits are empty before ordering cabinets?

That vertical plank you're seeing is almost certainly just a support hanger or some blocking used to hold up the soffit framing itself. When builders put these boxes in, they need something to nail the bottom plate to, so they drop down scraps of wood from the upper joists to create a ladder frame. It doesn't usually signify anything structural or hidden utilities, just the skeleton of the drywall box you're looking at.

In a square kitchen where the soffits only run on two walls, there is a decent chance they were installed purely for aesthetics or to house those recessed lights. However, the risk factor depends entirely on what is directly above that kitchen. If there is a bathroom on the second floor right over those cabinets, you need to be extremely cautious. Builders love using soffits to hide horizontal drain runs or vent stacks that need to bypass a joist. If you see a 3-inch PVC pipe, your plan for ceiling-height cabinets just got alot more expensive.

I relly wouldn't recommed ordering those cabinets til you've done more than just peeking through a light fixture hole. A recessed light opening only gives you a limited field of vision. Go get a cheap borescope or inspection camera that plugs into your phone. Drill a few small half-inch holes at different points along the run and snake the camera in there to see the full length. It takes ten minutes and can save you a nightmare scenario where you have thousands of dollars in cabinets sitting in your garage that physically won't fit. If the space truly is empty except for some basic wiring, moving that electrical is a breeze for any halfway decent sparky. But if you find a main HVAC trunk line or a plumbing stack, you'll have to decide if you want to pay to reroute those utilities or change your cabinet order to include a smaller crown molding or a shorter upper. Better to delay the order by three days and be certain than to guess and lose your shirt on a restocking fee.

I've heard horror stories about people finding massive bulkheads that are actually part of the house's main support beam instead of just a hollow box. If I find somethig like that after I poke a hole, can a kitchen designer ussually hide it with a filler panel or am I stuck with short cabinets forever? I'm also wondering if those vertical hangers are always just scrap wood. My neighbor found a steel bracket in his and had to stop everything becuase it was actually holding up a heavy light fixture. Just makes me wonder how much weight those little boards can actually handle before the ceiling starts sagging.

Watch out for small diameter gas lines or flexible HVAC ducting. Even if the main stack isn't there, builders often snake a 1-inch gas line for an upstairs fireplace or laundry right thru those boxes becuase it's easier than drilling joists. If it's gas, moving it isn't a quick DIY fix—you'll need a licensed plumber. Also, check if that vertical board is part of a manufactured floor truss. If the soffit is integrated into the structural web, cutting it is a nightmare. Better to pull down a whole sheet of drywall now than to guess.

One thing people forget is that even if it's 'empty,' you might find a mess of junction boxes tucked away where you can't see them from the light hole. I ripped mine out last year and found a hidden junction box for the entire kitchen circuit buried in there. It wasn't a dealbreaker, but it meant I couldn't just shove the wires up into the ceiling without some real electrical work.

Also, be prepared for a ton of construction debris—I found old soda cans and literal handfuls of sawdust from the 80s sitting on the drywall. It’s a messy job, so mask up before you pull that first piece of rock down.

Don’t forget to check if your range hood vents trough that soffit. If your stove is on one of those walls, there might be a 6-inch rigid or flex duct running horizontally to an exterior wall. Moving that usally requires cutting into the exterior siding or brick, which is a total pain. Also, don't assume the drywall behind the soffit is finished. Many builders leave the ceiling and wall bare behind those boxes to save money on mud and tape. You might have to patch a massive gap in the ceiling and wall texture before the new cabinets can even go up. It adds a ton of prep work.