Most MMDVM Hotspots use STM32F103's in them.
https://github.com/juribeparada/MMDVM_HS
The MCHF . HF SDR transceiver uses the STM32F4
https://github.com/df8oe/UHSDR
Most of the TYT hand held VHF/UHF transceivers, like the TYT MD-380 use the STM32F4. There isn't fully open source firmware for this, but there there is the MD-380 tools which is an open source software patching system which adds a load of additional functionality to the MD-380
https://github.com/travisgoodspeed/md380tools
In the longer term, I'm hoping to port the OpenGD77 project (which runs on GD77 / TYT MD-760 radio) to the TYT MD-380 etc, but the MD-760 uses the NXP MK22 MCU, which is a cortex M4 processor, but unfortunately not a STM processor
I use the STM32F103 all the time in my Ham Radio projects. I designed a pseudo VFO add-on for a Codan HF transceiver, which had a rotary encoder as it input and fooled the Codan into thinking that the operator was pressing the UP and DOWN buttons really quickly.
Also by messing around with the main PLL and one of the timer divider, I managed to produce the BFO frequency required by lower sideband, as most Codans don't come with both the upper and lower sideband BFO crystals.
The frequency of the BFO was off by about 30Hz, so I just pulled the master crystal on the BluePill a little with a capacitor.
I've also just built a mini VFO for a Pixie QRP single board transceiver, which uses the STM32 as the controller and uses a Si5351 as the actual RF generator. This includes a mini OLED display and a rotary encoder etc.
A while ago, I attempted to convert a UV-82 VHF/VHF handheld into a mini repeater, and I had to remove the original, unknown MCU, and connect a BluePill to control the transceiver chip (AT-1846S) etc.
Someone at my local radio club, programmed a BluePill to generate Morse code for tuition purposes, with various modes e.f. 5 figure groups, at various speeds
And I'm just in the process of trying to get an old Philips UHF 815 repeater working again, and I'll be using a BluePill as the main control board, and will be interfacing it to a Si5351 as the RF generator for both the Tx and Rx units to replace the crystals.