So what does this mean, functionally/technically? Are they now un-indexed from Google searches? Unable to show in "top" posts? What is the effect?

Edit: found it.

>Quarantined communities will display a warning that requires users to explicitly opt-in to viewing the content. They generate no revenue, do not appear in non-subscription-based feeds (eg Popular), and are not included in search or recommendations. Reddit may also enforce a number of additional product restrictions that exist currently or as they may develop in the future (eg removing custom styling tools).

Edit x2: I took a look at the source code on t_d, and they now have a

<meta name="robots" content="noindex,nofollow" />

tag. Per the Wayback Machine they didn't have this tag yesterday. So if I'm reading things right, they are now no longer indexable by search engines like Google.