apex – SOQL query and record Id in update DML. Did behaviour change?

As pointed out in other questions some fields are queried in APEX even if not mentioned explicitly, including Id field.
I can remember these fields could be used only for reading data, not for actually updating records, meaning you had to explicitly query Id field to update a record, but you could read Id value without querying it.

So code like this would throw an error:

Account acc = [SELECT Name FROM Account Limit 1];
acc.Name="Test";
update acc; // Error - SObject row was retrieved via SOQL without... or something like that

But code like this would not throw an error:

Account acc = [SELECT Name FROM Account Limit 1];
System.debug(acc.Id);

But it is no longer the case. Or it never was?
Help me with this I’m losing mind over this!