During Legion Blizzard made it so that ‘old content’ areas had no sharding on RP realms, only the broken isles, the broken shore and Argus had sharding, but only with other RP realms.
I have a feeling that Boralus works the same way for RP servers now. Warmode might change that though, since I heard some people from AD saying that they see people from non-RP servers when they have WM on.