Is this still true? It seems like most IoT stuff has chips that easily eclipse the capabilities of my Amiga1000 and are 32bitbwith hardware floats. Looking around, I'm having trouble finding something that isn't a 32bitHF in my house. Possibly my Keurig machine? Even using digital instead of an RLC implementation in a coffee machine seems like a waste, but I'm old.
Cortex-M4/M33 are pretty widely deployed on microcontrollers with control hardware, for example the STM32G4 and Reneasas RA series.
There's still a few reasons not to use it, though. Saving and restoring the float registers adds interrupt handling overhead. If you can fit into 16 bit integers, you can also use the 2 way SIMD instructions and get at least double the throughput.
Finally, floats themselves have a lot of footguns. You have to be very careful about propagating NaNs etc into control loops. It's also really easy (in C) to accidentally promote to double.
Even if you could, floating point may not be the best representation for control applications. There are a lot of problems, like NaN, as somebody pointed out. Even neglecting that, a fixed point representation may be better.
You don't need the fine precision or large range that floats have (we get those using exponents). Signal variations below a certain threshold are drowned out by the noise floor. Signals in control systems also have an expected maximum bound.
At other end, stability calculations based on quantization noise may be difficult with floats, since their least count changes with exponent. Fixed points have a uniform LC throughout its range.
Most PID applications are not IoT. Many use a cortex M0 which doesn't have hardware float support, or non Arm, even 8bit. When you are selling millions it pays to use the cheapest part you can get away with.
What do those things go in anymore? Surely there are asic pid controllers that are insanely cheap? Tuning an analog one I can see probably doesn't make sense anymore, but there is something nice about being able to do that with a screwdriver instead of jtag. What do these M0s go into?
I used an 8-bit PIC micro a couple years ago for power applications (think non-IoT lighting). The specific microcontroller we used had nice peripherals for sensing, and controlling diodes, but no FPU. I remember looking into getting something external to handle the PID, but the cost and board layout constraints made it challenging.
