The "Alterations to Magic" section on page 24 of the Curse of Strahd adventure mostly describes how teleportation and planar travel are affected by the Barovia demiplane. The "Cosmetic Spell Modifications" section, which is the part your question refers to, states (emphasis mine):
At your discretion, a spell can be modified cosmetically to enhance the horrific atmosphere. Here are examples:
In other words, the spell modifications listed are just some examples for optional rules, not RAW that you must abide to, and a DM running this adventure is free to use all, some or none of these suggestions in addition to any number of other cosmetic modifications they care to make.
This means that find greater steed may be affected in this way if the DM chooses to do so, but even find steed doesn't have to be affected at all in the first place. It's all up to the DM.
It's also worth noting that find greater steed was released in Xanathar's Guide to Everything, which came out after Curse of Strahd, so there'd have been no way to include it in Curse of Strahd anyway.
Furthermore, even if the spell did exist before, it's a 4th-level spell, only available to 13th level paladins or 10th level bards, and given that the adventure only goes up to level 10, the designers may have chosen to leave it out as it would be unlikely to come up anyway, especially considering that they had already included find steed, which is already an adequate example to guide the DM on how to cosmetically modify such spells (and again, the spells listed are only examples and suggestions, not an exclusive list).