To my recollection, there’s never been a grain slider in LR Mobile, it’s only been available as a slider via the desktop app (now Lightroom Classic).
Grain is available to LR Mobile as a preset. If you go to the Presets section, change the category to “Components,” you’ll find three grain presets available. While the organization of the presets is a little different, this is the only way I ever recall being able to add grain in LR Mobile.
As I said, I used the grain slider (now removed) in about every photo I edited on Lightroom CC for iOS in 2017. I do not use the desktop app. There was a slider, as I said, under effects, and I'm asking why it was removed. The functionality is still clearly present (see my original post), so why was the UI/slider removed?