The multiplayer playlists for Halo: The Master Chief Collection have to operate across not only different maps and game modes but the games themselves. That seems to have caused some extra issues for the final release, as : Combat Evolved-especific playlists won't be available at launch, while other changes to the existing playlists will exclude some games and maps.
"We are holding on introducing the dedicated Halo: CE playlist. Up until this very day, we’ve been making (and are continuing to make) investments to ensure that Halo: CE’s online/network performance is the best it can be within Halo: The Master Chief Collection, and we’re committed to making additional investments to make this happen," the official blog posts stated.
"The current Halo: CE experience will be playable in custom games, as well as the Team Slayer and Big Team Battle playlists, but we are holding on the dedicated playlist until we are able to provide the best experience possible. Lastly, due to your feedback over the past few weeks, we’ll be moving the Halo: CE playlist to 2v2 when it launches."
There are other cross-game playlists that will be updated post-launch. "The Team Hardcore playlist will launch only with Halo 2 Classic tournament-approved game types," the post said.
"Behind the scenes, we’re working to integrate Halo 3’s custom map variant files into the playlist, and they aren’t quite ready just yet. As custom map layouts are extremely important to this playlist and community, we’ll be adding the Halo 3 game types into the list when they can be fully integrated, tested, and brought in."
You can read the entire post for an updated list of what will, or won't, be included in each playlist.