Vanilla mobs have/(had? can't confirm in latest update due to the platform) do_not_upgrade enabled by default.
This was noticed when a past update was released that changed the vanilla behavior of new mobs generated, but the behavior of mobs from before the update remained unchanged.
Comments 3
Everything said here is based off my limited understanding for this component.
There aren't many examples of mob behaviors being changed by updates that I know of. (I suspect that it is no longer enabled by default, as I noticed that old spawns of the hostile Phantom seemed to attack despite being from the update where they didn't. Though it is also possible that the Phantom has the component disabled already). Any issues caused by this component would be temporary, once the mob(s) in question despawn or are killed, no mobs will have the issue. But now knowing that such a component exists, I don't know why it should or would be enabled for vanilla mobs in vanilla worlds. I can't check for sure to see if this component is still enabled by default, I just know that it was at some time for a handful of mobs.
Hypothetical (this is assuming that the component is still set this way, and is set this way for all vanilla mobs);
The behavior of a tamable mob gets changed. Say it has some new ability. When players would go to try the new ability they would find out that they are unable to because their version of the mob spawned before the update that added the ability.
Or say there's a bug involving an invincible hostile mob. The old spawns of the mobs would remain invincible after the bug fix.
A past example was where there was an issue that caused Magma Cubes to take fall damage from their own jumps. After the bug had been fixed, any Magma Cubes created before the fix were still taking fall damage. (Though the problem lasted for about 10 seconds thanks to their own jumps eventually killing them, thus this example quickly resolved itself in a somewhat morbid way).
The component doesn't cause any problems until something relating to mob behavior is changed. (Or it keeps old problems if a fix is prevented)
In the interest of avoiding future issues (and in the interest of utilizing the ability to disable the component in the only setting where it would likely be used), it would be preferable if this component was disabled by default if it isn't already.
Cleaning up old tickets: This ticket has not been updated recently (~7 months+) so is being closed as Cannot Reproduce. If you feel this is still a valid issue and is affecting the most recent versions, then please comment, or create a new ticket following the Issue Guidelines.
Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki
Are you able to add some examples of how this is affecting mobs specifically and how this is affecting gameplay?