OldSteve:
The issue that I described happened with avrdude 6.0.1. That was using the "libusb-win32 v1.2.6.0" driver installed using Zadig, (installed to get USBASP working with Win10), along with IDE V1.6.9 and "AVR Arduino Boards" V1.6.12.
Are you sure? Arduino AVR Boards 1.6.12 does install avrdude 6.3.0-arduino2 and cause it to be used by the Arduino IDE but if you have 3rd party AVR boards installed via Boards Manager it may cause avrdude 6.0.1 to be used instead. Things are a real mess right now.
OldSteve:
It won't become a problem for me until I update to IDE V1.6.10, which I can't do until DrAzzy's ATtiny cores are compatible with it and "Boards Manager".
We did make a fix to the ATTinyCore Boards Manager file so it shouldn't cause any problems with Arduino IDE 1.6.10 now. As to whether the new versions of avr-gcc and avrdude will cause issues with that core, I'm not sure.