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anyone tried mixing for example aluminium head and cast iron engine block

Is there somewhere you're going with this, by any chance?

Mixing materials is not an uncommon practice in real life, either. Coincidentally, the RB28DETT engines have a cast iron block and aluminium heads.

What I'd like to know is that I'm pretty sure there used to be a pretty hefty reliability penalty for mixing materials. Did I miss a conscious decision to phase this out? Because these days I don't seem to observe any difference regardless of the combination.

Try doing it in the 70's or 80's or 90's, it's been reduced but not removed.

Well yes, it has been done a million times in real life. The game has this slightly odd mechanic that massively reduces the reliability of engines using different materials for block and head due to the materials reacting differently to the heat produced by the engine. I don't know why you're asking this but I guess that the reliability penalty kind of limits an engines capabilities.

Yeah, like nialloftara said, it reduces through the decades. By the 90s, it is a pretty low penalty now.

FYI, mixed metal engines were notorious for head gasket issues in the 50s-70s