Search chinese websites for “FX2N” and you get lots of cheap PLC boards with an STM32F103 on them, and they even have break-out pads for re-programming the STM32.

STMicroelectronics is extending its range of STM32 Nucleo development boards with scalable, small-form-factor variants to support the 32-pin members of its STM32 microcontroller family. The STM32 ...

The 50MHz oscillator is provided on the board, but is “optional” (you decide if populate it or not) because the STM32 is used as 36/8MHz clock generator (in the photo it isn’t populated).