I'm in Delta but still haven't received the Mythic Achievements update

Update: I have all the maps except MYTHIC showing now, and I just downloaded mythic again. Yet the mythic maps wont still show. I've cleared cache, and I'm out of ideas.

The game is older and graphically outdated since these new games are updated much more. It is true that the game is like a "ghost town", there's hardly anyone that plays Team Mythic and I need one last achievement for the Mythic Map Pack. There's only a
few thousand of people left playing it, but soon it will be hundreds.

Well my Pillar of Autumn achievement finally unlocked today. Still waiting for Delta Halo and Prophet's Bane

Hi Luna!

I've definitely had a similar situation before come up, usually around 3am pacific time they will roll out Insiders Updates. One of the nice things about being an Insider is getting access to frequent updates and new features, but it can get annoying if
you're receiving too many updates right when you need to be online. What I would suggest if perhaps lowering your Preview Ring tier. The tiers are as follows :

• Alpha – Open on an “invite only” basis and receives updates at the earliest point in the preview window.
• Beta – Open to Xbox Insiders with a Program tenure of 3 or more months and an Xbox Insider XP Level of 5 or higher. This ring receives updates shortly after Alpha.
• Delta – Open to any Xbox Insider with a tenure of 1 or more months that has reached Xbox Insider Level 2 or higher. Depending on certain edge cases where further attention is required from Alpha or Beta rings, Delta Insiders may receive
  system updates before Beta, though this is not a common scenario. Additionally, while early system updates are not guaranteed across the Delta ring, all Delta Insiders will receive the update prior to GA.
• Omega – Open to anyone and receives system updates shortly before the GA (General Availability) release.

If you are currently on Alpha or Beta, maybe lower yourself down to Delta or Omega? Those rings receive far less frequent updates, but still get some nice goodies ahead of everyone else. On Alpha I receive updates sometimes daily, but Delta and Omega receive
them more like weekly or monthly.

You can make this change within the Xbox Insiders App on your Xbox, going to "Insider Content" then head to "Xbox One Update Preview" under "System" and select "Manage. If you'd like more information on this, check the link below. Cheers!

https://support.xbox.com/en-US/xbox-one/console/xbox-insider-program

Seems you have to be invited to join the Alpha ring

https://support.xbox.com/en-US/xbox-one/console/xbox-insider-program

The current Xbox One Update Preview rings are:

• Alpha – Open on an “invite only” basis and receives updates at the earliest point in the preview window.
• Beta – Open to Xbox Insiders with a Program tenure of 3 or more months and an Xbox Insider XP Level of 5 or higher. This ring receives updates shortly after Alpha.
• Delta – Open to any Xbox Insider with a tenure of 1 or more months that has reached Xbox Insider Level 2 or higher. Depending on certain edge cases where further attention is required from Alpha or Beta rings, Delta Insiders may receive
  system updates before Beta, though this is not a common scenario. Additionally, while early system updates are not guaranteed across the Delta ring, all Delta Insiders will receive the update prior to GA.
• Omega – Open to anyone and receives system updates shortly before the GA (General Availability) release.