ATTiny85/Digispark why P5 is always 4.7V?

Digispark should have all five pins available for GPIO. But someow P5 is always high for me and shows 4.7V on multimeter, even if I set pinMode(5, OUTPUT); digitalWrite(5, LOW); Moreover, if I measure P5 to the ground, P3 (or P2) temporary becomes high as well. So is P5 really available or not?

I suspect your P5 is still configured as the reset pin and the RSTDISBL bit is a logic 1.
Putting your multimeter between PB5 and GND, may pull PB5 low, causing a reset.

You can try by bridging PB5 to GND via a resistor and then observe if it resets.

Thank you @hmeijdam , how should I change the P5 role to make it a general-purpose pin?

You need an ISP programmer for that like a USBASP dongle.
And then you can check the value of the fuses with a program like avrdudess
Word of warning, you can make your T85 inaccessible if you set fuses wrong, and then would need a 12volt HV programmer to set it back to it defaults. Especially after changing RSTDISBL is set to "0" even your USBASP programmer cannot program it anymore.

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Be aware that after changing P5 you can't upload software to your Digispark with usual way. You will need a speciav HV programmer

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