The problem is not with Squarespace URLs - it's actually with pages that DON'T exist on the Squarespace site, but DID exist on the prior site.
Case in point: we've moved multiple clients from WordPress to Squarespace. In each case, old WP URLs are actively being crawled by Google to this day. We need the option to disallow Google from crawling folders like /wp-includes/ and /wp-content/ because that content is simply no longer there.
P.S.
A 301 redirect is not an appropriate solution here because the old URLs are spammy content caused by a hack of the old website. Squarespace does not offer an option to set a 410 server response on an individual URL level either (another feature Squarespace should absolutely offer to site administrators).