Weirdly the word "fossil" in there is just reminder text.

Question regarding how fossil turns work inspired by that interaction: If you attack a player during a fossil turn, can they block with any creature? Only fossil creatures? Do all fossil cards get a free dose of shadow when used against a deck outside the set?

Maybe you wanna collect a FAQ somewhere. ^^

I thought I made it clear that non-fossil permanents aren't considered existing during the fossil turns on the page for fossil turns.

"During a fossil turn, only fossil permanents on the battlefield are recognized. Permanents without the fossil supertype are not considered present. Similarly, only spells with the fossil supertype may be cast or otherwise used during the fossil turn. Only fossil lands may be played. Treat all cards on the battlefield without the fossil supertype as if they weren't in play during each player's fossil turn."

Well, it's always possible that you clearly communicated something you didn't intent. A good deal of time I look at custom cards I deal with people not knowing what the actual text they've written means.

And since having a major shadow-like gameplay situation doesn't seem like a particular gameplay one would want to that extent I wondered whether you actually considered this or maybe didn't take things into account and just wanted the active player restricted to their fossil permanents.

Sometimes a FAQ contains the question "Are you sure about XXX?" :)

So technically this doesn't need to say "fossil" since creature tokens that are not fossil don't exist?

I don't know whether writing it out is clearing up things or making things clearer.

I don't think you're wrong since non-fossil tokens wouldn't be present during a fossil turn. I was just constantly using fossil in the text because I thought I was being as clear as possible.