I thought the "Leader Zombie" was something implemented by Notch, but it was bugged and didn't work properly?
Either way it is interesting to add it/choose to fix it now.
I remember reading about it years ago, supposedly it was a "hard" difficulty mechanic for a few years but hardly anyone has ever heard of it. Most people never heard of it because of how rare it is, because the chances of someone having the game set to "hard" while also having the right
regional difficulty (something that most people also don't know about). Regional Difficulty scales super slowly and by chunk, meaning you won't see a noticeable difference until you spend dozens of days in a single chunk. I think, I skimmed the article. It's also affected by the moon phase, with the full moon being the strongest.
Further reading suggests
any zombie that's hit has a chance to spawn reinforcements, not just leader zombies, but only on "hard" difficulty at a 10% chance. Leader Zombies however can range from 50% to 75%, this
stacks with "hard" difficulty adding the reinforcements to all zombies and so you can even get a 85% chance.
One of the main reasons this is a worthless mechanic is because there's a huge radius around the zombie where reinforcements can spawn, this includes deep underground, in fact I'm willing to bet most reinforcements will be underground due to the very low light level. Each zombie loses a percent chance to summon reinforce after each reinforcement, but of course this can be abused to
farm the shit out of zombies.
TL;DW zombies are lured into a drown trap with an armor stand, the trap will cause drowning damage (zombies drowning will always be hilarious to me since they're undead) and will cause the zombie to spawn more zombies pretty reliably due to only taking half a heart, but will decrease their reinforcement chance. Zombies that drown become...well, Drowned and have their reinforcement rerolled and spawn
normal zombies for some reason, reinforcements don't count towards the mob cap so you can get an ever increasing amount of zombies to your farm. This is the only farm I've seen in Minecraft that actually becomes
better over time.