OK, it's not actually as bad as it sounds. The "loot boxes" come from a new mod for ZDoom, a fan-made port that, among other things, allows you to play and alter the original Doom on modern computers.*
The so-called "Doom loot Box Mod" removes all weapons and power-ups from the map, replacing them with locked crates. You need a key to unlock each crate, and keys can be obtained either as a rare random drop from downed enemies or via in-game purchases. Read more...