Is it permitted to give shout-outs to authors on other webnovel platforms (eg. WN, ScribbleHub, WP), with a link to their work? If not, then should shout-outs be restricted to RR only, or can novels on other sites be referenced given that no link is added? Any other rules that shout-outs should follow?

Don't know why it wouldn't be permitted, but I think readers wouldn't like to be linked to a different site. I mean, if I clicked a link to a story and it took me to WattPad instead of staying here, I'd be fairly annoyed. Especially since I don't have a WattPad account. And if I clicked something here and it sent me to the advert-filled ScribbleHub, I'd never click a story link again without checking that it stays on RoyalRoad.

Definitely don't link to anything outside of RR in that regard.  Mentioning another fiction from outside RR if you don't say what website it's on is probably the most the staff would let you do, to be honest...

Mad Wrote: the advert-filled ScribbleHub

What!? Scribblehub had adverts!?

People link to amazon all the time, I don't see why not. But maybe mention "It's available on WebNovel!" before the link or something? 

Linking outside of the forum is against the rules, I do believe.... 

And sure, you can shout out whatever you want, but that doesn't necessarily mean anyone will go out of their way to search for it...