Not sure if thou hath found the idea in thy derriere, for I cannot recall whether or not Gooseby was working on this.
I hope the thread with the idea is scrapped myself, it was a bad thread. The worst one. 1 movement siege units is the very definition of "not fun" (check a dictionary, I'm not lying) and would warrant some city defense building reductions pretty much just a moment after all of them were buffed because cities would become too hard to get both for AI and player alike with just Walls.
It'd be better to just reduce Siege ranged strength while improving its city damage promo so its worse against units/archers, but remains cruel against cities. or just give it a promo granting it -X% damage against land units. Or just leave it as is.