Shiny locks didn't really become a thing until Pokemon Black and White, if I recall correctly. Until that point, there weren't cutscenes where the immersion could be broken by encountering a shiny legendary Pokemon. Which is still a really dumb reason to lock off their shiny forms.
I think Emerald had a really weird and specific RNG which makes it difficult if not impossible to get shinies most of the time? Your best bet for them would probably be Pokemon HeartGold or SoulSilver.