It might be good to keep all the ruby-related tags that have been whitelisted, even though rb and rp aren’t used as much anymore. rp is used for legacy support for old browser though, so maybe not particularly relevant for Discourse. No big deal one way or the other, just thought I should mention.
EDIT:
More importantly, when I test this on the word in my OP (and only that word) it misses the surrounding ruby tags, so it doesn’t format the text as expected. When I highlight text on either side it before quoting, includes the ruby tag, but that workaround wouldn’t work if the word was the first or last part of the post. Is there anything that can be done to make sure it includes the ruby tags?