Mineshafts state that they generate in 21w06a, when using the locate command. However, they rarely have actually generated. Only about 1 in 3 are present.

In addition, the mineshafts that do generate still do not have the chests generate, and also have the rails or spawners generate much less frequently and much smaller. In addition, all the mineshafts generate at about the same height, below Y=-34.

Seed used to test: 3393716354784051748

Floating Islands preset, no specific biome selected.

Coords:

• 480 480 – No mineshaft
• 1152 720 – No mineshaft
• 1152 592 – No mineshaft
• 1664 1808 – No mineshaft
• -1664 -1808 – Mineshaft present, but partially broken. No spawner present.
• -2256 -2384 – Mineshaft present, but no spawners present.
• -2720 -2736 – No mineshaft
• -3200 -3440 – Mineshaft, finally a spawner.
• -3744 -3808 – No mineshaft