I found two (Austrian) articles (in German; from a not unserious newspaper): https://derstandard.at/1271377233456/Nach-versaeumten-Hinflug-Urteil-untersagt-Airlines-Storno-von-Rueckflug https://derstandard.at/1358305364054/OGH-kippt-AUA-Hin--und-Rueckflugklausel
from 2010 and 2013, respectively. If I understand them correctly, some court in Austria ruled that Austrian Airlines and Lufthansa are not allowed to prevent passsengers, who missed the first flight, to board on the return flight. (The first article says that in this case, the passengers must pay the price difference to a one way ticket, the second article says that passengers do not have to pay the price difference anymore).
What is the situation today? Are there similar rulings in other (possibly EU) countries? So my question is: Which airlines allow (and under which circumstances) to use only the return flight?
Edit: I was told that this question might be far too broad to be allowed for this site and that I should reduce it. So I want to ask this question only for EU airlines (and afterwards I will probably open a question for non-EU airlines).