Maybe diggin a topic a bit, but as for June 2021 :

In library file MyHwSTM32.h one line seems to be lost :

#ifndef MyHwSTM32_h
#define MyHwSTM32_h

#include <IWatchdog.h>
#include <itoa.h>
#include <SPI.h>
#include <EEPROM.h>   <<--- this is necessary to compile agains BluePill

and something like this in platformio.ini

[env:bluepill_f103c8]
platform = ststm32
board = bluepill_f103c8
framework = arduino

upload_protocol = stlink
debug_port = stlink
debug_tool = stlink
monitor_speed = 115200
monitor_filters = time, default

Just to keep you entertained.
Cheerz.