DIY mini-split systems run most efficiently down to around -5 degrees C. They will continue to work down to around -15 degrees C but become less efficient at extreme temperatures – you can view the tech specs here.
MrCool Universal Series can heat effectively below freezing. It can even heat effectively below zero and keep heating effectively down into negative double digits. It can keep a 1500 sf house at 70 degrees in -20 and below – you can view the tech specs here.