Mod version: 1.19-7.1.7
Forge version: 43.2.0

spawn.json:

[
    {
        "result": "deny"
    }
]

Bees and structure mobs (e.g. in villages, ruins, or mansions) are still spawning. Everything else seems to be following the rule. I also tried adding a "deny" rule above the blanket deny for "spawner": true, but that didn't help. Tested on an instance with no other mods.

Add "onjoin": true

That worked, thank you! Sorry, could have sworn I tried that already.