Because even though they weren’t intended specifically to provide a change in gameplay, they did cause a change in gameplay. Spell Batching caused very unique situations to be possible (mages sheeping each other simultaneously, heals being able to save a tank even though they technically were killed 200-400ms ago). “Leeway” (still think this is a dumb term) caused attack ranges to be within a specific variance, resulting in a very different experience with/without when doing things like circle strafing.
While they may be “optional” in today’s server performance and connection throughput/reliability, they very much impacted gameplay of Vanilla. This is why, specific to Spell Batching, Blizzard has re-implemented the behavior of Spell Batching to recreate authentic Classic WoW. They have “commented” on this before, when they announced they were intentionally and specifically adding Spell Batching in.
While they have not specifically called out “Leeway” in the same way, it stands to reason it is implemented for the same reasons as Spell Batching.
TLDR: They’re both authentically classic, therefore, they are in Classic WoW.