I tried to set the spawn of the abyssal guardian so that it could spawn only in a small amount in a given biome. After adding the "biome" parameter, nothing changed with respect to the biome and it still respawns in other biomes.
spawn.json
[
{
"mob": ["aquaticabyssmod:abyssalguardian"],
"biome": "aquaticabyssmod:venttrench",
"perplayer": true,
"mincount": 40,
"result": "deny"
}
]
potentialspawn.json
[
{

	"mobs": [
		{
			"mob": "aquaticabyssmod:halibut",
			"weight": 1,
			"groupcountmin": 1,
			"groupcountmax": 1
		}
	]
},
{

	"mobs": [
		{
			"mob": "aquaticabyssmod:abyssalguardian",
			"weight": 1,
			"groupcountmin": 1,
			"groupcountmax": 1
		}
	]
}

]

You need to use the biome name in 1.12. i.e. Like 'Plains'

So in this case it will be "venttrench" instead of "aquaticabyssmod:venttrench" and it should work?

Probably 'Venttrench'. The case mattters. It's the actual name that you see when in F3 I think