For those of you that have deleted your New Player Experience (NPE) decks, or other special pre-constructed (pre-con) decks, you may find that you needed them back at some point. Here's a quick list of all of the per-con decks that have been released for Arena.
Please let me know if there are any missing so that I can keep this list updated.
Ravnica Guilds (7/12/19 - Current)
These are the current dual color NPE decks given to new users that completed the Account Mastery during the New Player Experience during the M20 core set.
On Sept 20, 2019, the dual colored decks were also given to all current users that had previously completed the Account Mastery prior to this date, which was the release of Throne of Eldraine.
Color Challenge (3/12/20 - current)
Starting March 17, 2020, the New Player Experience was revamped. These are the refreshed mono-color decks available through this system.
M20 Account Mastery (7/12/19 - 3/17/20)
Prior to March 17, 2020, these were the old mono-color decks available through the old Account Mastery system. They have been refreshed mid-rotation and are now part of the Color Challenge system for new players.
These decks were obtained through codes given out at various times through various places.
Historic New Player Experience (Closed Beta - 7/12/19)
These are the decks that were available from closed beta up until the release of M20 on July 12, 2019.
For further detailed information on these decks, check out the Magic Arena Wiki