Digispark: Driving high Zener-connected pin on ATtiny85
The IO pin has absolute maximum rating of 40mA. The digital output voltage rating is listed at 10mA current, which basically tells the drive strength. But pins are not guaranteed to source current greater than the listed condition. However, the characterisric curve shows output voltage vs output current up to 20mA.

So basically, the pin should typically be able to source 20mA at about 4.5V,  but they don't guarantee that it can source over 10mA.