The new 3.0.0 core release has indeed broken a lot of things (see e.g. here).
It might be better if you stay on the stable version before that to get running again, which would be platform version 2.6.3. So try the platformio.ini
[env:esp8266_d1mini]
; ESP8266 D1 MINI 4M bytes Flash buy from here https://amzn.to/3i1gPIz
; The MINI PRO version also works but is no longer required
; *** ESP8266 CORE for Arduino version 2.7.4
platform = espressif8266@2.6.3
board = d1_mini
upload_protocol = esptool
board_build.mcu = esp8266
board_build.f_cpu = 80000000L
upload_speed=250000
upload_port=COM4
board_build.filesystem = littlefs
(docs).
If that works you might want to look into getting up-to-date libraries and/or update the sketch code to work with the current 3.0.0 release.