The Shadowlands Vicious Saddle mounts will be purchasable in Dragonflight! This has been noted by MrGM on Twitter. It is currently unknown if this will be available right on launch on November 28, or at the beginning of Dragonflight Season 1 on December 12.*
* It is worth noting that on the Dragonflight Pre-Patch PTR, the Shadowlands Seasons 1-3 mounts are already available for purchase, but this is not the case on live servers.