https://wiki.openmw.org/index.php?title=Bethesda_Emails
But it's certainly unclear in the general case whether engines that require the user to provide their own (presumably legally purchased) data files are derivative works of the original engine. It's even more iffy when, as is the case for https://pokemmo.eu/ , it might be illegal in certain jurisdictions to obtain a ROM even if you have purchased the physical product. Of course the creators protect themselves legally, but is it ethical for a software product to effectively encourage users to break a law that is itself unethical?
(IANAL)