Since you mentioned it I had to look it up
and also ended up with a find on stereophile.com: In
their review of the CS3.6 they state that "the crossover is a true acoustic first-order (6dB/octave) type", though. As laid out by
@jackocleebrown this is pretty much possible, maybe unless you opt for a 5- or 6-way design, which leads to more problems than it solves.
By now I cannot imagine any serious speaker manufacturer (read: those who don't compare their products with musical instruments) not paying attention to the acoustic slope formed by the driver (in it's cabinet) and the applied filter. Not so much when it comes to hobbyist, even though the required tool set is available for little money. From my point of view the real change brought up by KEF has been the mindset on how to design a speaker. Designers had already been aware of the fact that text book filters do not work (for many reasons). But the key idea behind acoustic Butterworth filters was to not only take measurements of real drivers in real enclosures but to actively aim after a well defined target response for all drivers inside
and outside of the pass band instead of starting with generic filters and then tweaking them until the overall response fits (somehow).
However, back in the late 1970s/early 1980s KEF 3rd order aB filters more often than not consisted of 3rd order electrical filters that were optimized to give a clean 3rd order acoustic slope. This is different from more current designs where they would rather use the least steep electrical filters to reach the desired (individual) response. Technology has advanced a lot and today they surely have much more control over the natural roll-off of their drivers. Maybe one can say that Thiel and KEF may have come from different directions tackling a common goal.
Most of this is history now, Thiel for sure is. But speaker design to this day is still about dealing with compromises. Putting your finger on just one point (like selling "non-cheesy" boutique parts) is very easy and can even grant you a cult entourage, but it's rarely the way to real progress.