Minecraft:1.16.5
forge:36.2.8
mod version:5.2.1
description:I use these code in spawn.json to make wither skeleton only spawn on soul soil,but it doesn't work.Following the code.

 {
     "mob": "minecraft:wither_skeleton",
     "block":"minecraft:soul_soil",
     "result": "allow"
 },
 {
     "block": "minecraft:soul_soil",
     "result": "deny"
}

First describe what you mean by 'doesn't work'. Secondly that syntax is not correct. You are missing [ and ] around it

No wither skeleton spawn on soul soil,and this code just a slice of whole code,I use [ and ] around it .

[{
    "mod":"mana-and-artifice:hulking_zombie",
    "dimension": ["minecraft:the_nether","minecraft:the_end"],
    "result": "deny"
},
{
    "mod":"mana-and-artifice",
    "dimension": "minecraft:overworld",
    "minheight": 100,
    "result": "deny"
},
{
    "mob": "minecraft:wither_skeleton",
    "block":"minecraft:soul_soil",
    "result": "allow"
},
{
    "mob":"minecarft:wither_skeleton",
    "result": "deny"
}
]

Note that spawn.json cannot add spawns. It can only restrict them. If wither skeletons didn't spawn on soul soil before then adding an 'allow' rule will not fix that. You need to add spawning rules to spawner.json in combination with the rules above

Understand.My bad,thank you.