Did English ever have an informal version of “we”

I don't believe English has ever systematically encoded formality in the 1st person plural. There are of course constructions like "us lot", but thes paraphrases aren't grammaticalising formality as such and their use is always completely optional.

What you might look at is the dual forms of pronouns that English used to have (so there was once a difference between "wit" = "you and me, we the two of us", and "we" = "we all as a group"). By pure statistics, it's conceivable (but I've no idea if this was actually the case or what data is available) that the dual forms occurred more often in "intimate" settings and so were 'de facto' informal forms in some sense.

Also consider the practice of using "one" as an honorific alternative to "I" or "we".