Lights on below 18,000’?
 

I understood this to be due to the possibility of pressurised GA traffic up to 18,000’.

This is not in the manuals for my European airline so would be really useful to have a reference / source.

https://www.pprune.org/atc-issues/25...-18000-ft.html

check this one- nevertheless when in Rome…
and that means most US-airlines have the SOP to turn on/off the lights going through 18. But you will be fine switching the lights at 10.

Most of the US you will be in Class E airspace below 18000, so there is the possibility of uncontrolled VFR traffic not talking to anyone if they are in VMC. Hence lights are a good idea.
Class E is controlled airspace, so if IMC prevail, everyone should be under ATC and talking. There isn’t much class G uncontrolled airspace in the US if I remember correctly.

What is the the connection with pressurization? I have often flown unpressurized aircraft, some with no transponder or ADS-B Out, up to 18,000 ft and much higher a few times.

(Before anyone jumps on "more than 18,000 ft without a transponder" research "glider wave window").