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While true, this is not officially supported as part of the RnRS standard I believe. They set these characters aside as reserved for future use.

Yes, you're correct. R7RS at least explicitly reserves brackets and braces, and only acknowledges parenthesis in its syntax.

I think this depends on which RnRS you look at.

Do some of the older ones just completely omit reserving them?

R6RS: pairs and lists, enclosed by ( ) or [ ] (see section 4.3.2)

Ah the problem child. :) I've mostly only paid attention to R5RS and R7RS.