The Original Mega Man X run has a sub category of Any% No Buster. I was planning on running this category on Legacy Collection because that's the version I have. However this category does not exists in this version. Is there a reason for this?

The Mega Man community has its own managed leaderboard over at:

https://megamanleaderboards.net/

That leaderboard is routinely reviewed for submitted runs. A moderator here would most likely need to be contacted directly to request a change on SRcom since it is not actively reviewed.