How come doom eternal doesn't have a Legacy Leaderboard but doom 2016 has?

I would assume it's because the game just came out and further categories are still being explored / discussed before adding them. I think the Discord has tons of discussion about stuff like that, especially since it's where people are mainly directed to when asking questions like these.

Just like Aussie said, categories are not set in stone and game just came out (leaderboards just opened 2 days ago). As for Legacy/No Major Glitches : people are already routing/running it, so I would expect this category to be around here somewhere in the future. ;)