the main alternative for object-unions was round parens () which are url-safe, but i was worried they'd be more likely to cause problems with URL detection -- markdown uses () as part of link notation for example.
i'm not actually sure how tolerant URLs are of literal curlies -- maybe it's mostly fine?
it's also not like super-critical that the syntax is all URL-allowed, worst case it just needs to be encoded (which it should be anyway)
...i liked curlies because they are a bit more visually distinct, and in regex they are a modifier of the previous thing.