Many airlines in Europe (easyjet for example) require you put in passport / ID details either when booking or checking in.

They require you to put the details in, but they are not required to check your ID when checking-in or boarding.

I’ve flown several times in Schengen where nobody checked my ID at all.

> Yeah but you need to provide proof of ID and the like if you book a flight

This was the initial thought, so seems like you agree.

You don’t even need an ID to board your flight in many airports for flights within Schengen.

Did you pay as you boarded? Do not assume the same rules will apply when there is a dramatic change.

If a company isn't going to charge until boarding, and will charge a fee for no shows then you can be certain that they've collected identification and a payment method. Otherwise, how would they charge no shows?