The line between SD and planing is often pretty blurry. If you look at them, plenty of "SD" hulls are just planing hulls with big keels (and therefore lots of drag) and not enough power to really get all the way on plane. As far as I'm concerned, if it accelerates through a range where it's plowing, bow starts to climb towards the sky, then you hit a speed where the bow drops, wake cleans up and the boat is on plane, it's a planing hull regardless of what it looks like underwater.
Many early planing hulls had keels, but they've typically gotten smaller or disappeared over time as boats have gotten faster. If you want to go faster, the drag from the keel is an issue and at higher speeds, it can be a handling liability. If you've got a deep V, there's less reason to have a keel in the first place, so they rarely have one.
Good point. Many SD hulls do indeed look like a plaining hull with a keel stuck on. Naval architecture has equations to calculate the amount of hydrodynamic lift (per pound of weight, etc.) and therefore efficiency of a SD design, though for most people it seems to boil down do, how fast will it go, and how big of an engine will it take to do it?
Even the definition of 'planing' can be blurry. Stick 5 x 600 hp mega-outboard motors on a Kady-Krogen 42 and it will probably climb up on to a quasi-mushy plane of sorts, but it won't be very efficient (or pretty) doing it. Almost any boat that exceeds hull speed (straightforward to calculate) is achieving a degree of dynamic lift to do it, though with varying efficiency.
Even that is blurry. Boats of the 1920's/1930's were more efficient than most modern designs. The engines back then were less efficient, had less power, so the designers had to be more efficient with the limited power they had to work with. The result were long, lean, narrow hulls that went relatively fast with modest power, and cleaved through the water fairly well. Those styles went away after the 1950's when higher power engines made it possible to have beamy boats that delivered wide cabins and the boat-show-wow-factor of cavernous interior spaces.
The Andreyale is a rare example of a pre-war narrow design currently built. To my eyes it's beautiful, but doesn't offer much cabin space for its length -
https://www.tofinou.com/en/motor-yachts/andreyale-15-m
https://i.pinimg.com/736x/1e/d8/c3/1ed8c352964e04428a2be9ead8709240--classic-yachts-motor-boats.jpg
Shannon had a similar idea with their 'SRD', Shannon Reverse Deadrise, hull design. I've been underway on one, and it also doesn't have the typical planing hull climb-over-the-bow-wave ride, but more smoothly transitions to speeds in the low 20's knots. Like the Andreyale, it's a narrow hull with limited interior space for its length (which I suspect is why it never sold well) -
https://shannonyachts.com/srd-technology.html
Occasionally you see something that to my non-expert, non-naval architect trained eyes seems like a nice compromise. The 'keel form' hull of the Camano 31 and 41 (the 31 is now being built by Helmsman) reportedly doesn't have much of a bow wave and doesn't so much 'come on plane' as it seems to smoothly transition from 7 to 15(ish) knots -
The old engineering maxim, 'Good-Fast-Cheap - pick any two' seems to apply to boats as well (Fast-Seaworthy-Roomy - pick any two). The typical 'downeast' style of SD hull, with a deep forefoot to cut through nasty seas, transitioning to flat sections aft to provide some lift, with a long full keel (and either soft or hard chine) seems to be a SD design that's stood the test of time, been battle-tested by scores of fishermen who go out in all kinds of nasty conditions when we'd stay in port, and provides more speed than FD but better seakeeping abilities than most planing designs.
Like most things in life, it's a compromise, with different solutions for different people.