Why any modern image spec should have a maximum resolution is beyond me.
WebP is based on VP8, which is a video format. Video formats can have maximum resolutions to make it easier to implement hardware decoders.
What application? That's 8x 8K or 32x 4K. Even theoretical futuristic giant displays would not need that.
Many smartphones now have 200MP e.g. allowing to crop, in astronomy they count in gigapixels ...
A 200MP smartphone is a nonsensical idea, their lenses are not big enough to utilize that many pixels. Even if it was actually possible, that 200MP is getting auto compressed the moment you message or upload it anywhere. I'm asking about legitimate uses.