A lot of spells that are accessible to players in-game are currently missing in the API, they include mounts, racial skills, passives, class spells, etc.
Some examples:
292361: Embrace of Pa’ku - Zandalari racial
6603: Auto Attack - Druid General spells
83958: Mobile Banking - Guild bonus
125439: Revive Battle Pets - Battle pets skill
233368: Broken Isles Pathfinder - Passive effect
78633: Mount Up - Guild bonus
34091: Artisan Riding - Passive riding
76275: Armor Skills - Druid General skills
197625: Moonkin Form - From Druid Balance Affinity talent
197628: Starfire - From Druid Balance Affinity talent
197626: Starsurge - From Druid Balance Affinity talent
225787: Sign of the Warrior - Calendar bonus
2108: Leatherworking - Profession
8613: Skinning - Profession
2550: Cooking - Profession
818: Cooking Fire - Profession skill
121820: Obsidian Nightwing - Mount
93644: Kor’kron Annihilator - Mount
59571: Twilight Drake - Mount
321406: Virulent Explosion - Doctor Iktus boss spell, accesible in the Adventure Guide.
320145: Buzz-Saw - The Manastorms boss spell, accesible in the Adventure Guide.
And that is only a part of the currently unaccessible spells as they are a lot more.
All spells were correctly accessible on the API at the beginning of this year. I can’t see this as something intended as many of the missing IDs are REAL spells learnt through different ways on the game.