It is NOT universally required. Incidentally, the FSF may well go after getting that.
That would severely limit where your device can be sold and used, though.
Intentional emitters of signals have to be certified if using a licensed band. That is the law in basically every country around - even tin-pot dictatorships don't want you emitting RF they don't know about.
Now, there are bands the FSF is free to use an unlicensed transmitter on - bands where the owners require licensing (i.e., the owners take on the job of certifying the design) like ham radio. Or the unlicensed bands, like 2.4, 5 and 6 GHz ISM. Here the regulations are basically along the lines of "do not have spurious emissions in the licensed bands next door".
The problem for the FSF is basically they can get it done, but then no one would touch it. If you wanted to produce a device and used the firmware, you likely couldn't put any blocks on what firmware your device runs (GPLv3), which means you cannot get your product certified because you cannot guarantee someone will run authorized firmware. You can put in hardware limiters (i.e., filters), but those things generally make your product perform worse.
The FSF is right on one thing - the firmware blobs are basically identical across users of the same chipset. The simple reason is that the chipset was certified with that firmware already, so if you want the least amount of trouble, you keep the firmware and hardware design the same so you can get through the gauntlet of certifications with the least amount of trouble. (They aren't cheap, you're looking at 5 and 6 figure costs just to do one certification pass so you want to minimize the number of passes you do. And for that you need to hire pre-certification labs which can do the runs and provide the analysis on what you need to fix before you run the gauntlet.
People do sell cellular chips as modules where the RF stuff is done on the module itself which reduces your need for certification and firmware blobs, but phone makers don't use them as generally they are more expensive overall. Chips from Qualcomm and such integrate everything in the SoC from WiFi and Bluetooth to the cellular RF into the main processor so all you need are some RF front ends which means you need only a minimal amount of extra chips. And those chips rely on firmware blobs to be loaded on boot, and those blobs are often binary to the vendor as well. (A place I worked at we had source code access to those blobs, it wasn't pretty).