The bug

The stats.txt file of /debug report contains spawn_count.misc, however that value is always 0 because it is not being counted.
Since 20w18a this information is also shown on the debug screen as last "M" value in the line starting with "SC" above the line with "Sound". And there it also always shows 0.

20w18a, Mojang names:
net.minecraft.world.level.NaturalSpawner.createState(int, Iterable<Entity>, ChunkGetter) ignores mobs of category "misc".
Maybe this could be solved cleanly by adding a method boolean isCountingMobs() to MobCategory and then only include the mob categories on the debug screen and in the debug report where the method returns true.

Reproduction steps

1. Open or create a world with no natural mob spawning (e.g. "The Void")
2. Open the debug screen (press F3)
3. Look at the last "M:" in the line above "Sound"
4. Spawn some villagers
   The "M" value did not increase and is still at 0