While newer chips like the F4 or G4 series exist, the STM32F103 remains the perfect teaching tool for three reasons:
For those learning to design embedded systems with this chip, several highly-rated educational texts and PDF guides focus on the "Blue Pill" development board. Google Books Programming Languages: Mastery of both the stm32f103 arm microcontroller and embedded systems pdf
You might ask, "Why learn on the F103? Isn't it old?" While newer chips like the F4 or G4