Side note: is this sample even working?

In scene 3, If I use MPPM and set the latency to 9999 ms, I still see the prefabs spawning instantly on the client. I'd expect them to spawn 9.99 seconds later.

Yes, it's still working.
Inside of DynamicPrefabLoadingUtilities.cs, it's stated:

    /// Artificial delay to the loading of a network prefab is disabled by default. To enable it, make sure to add
    /// ENABLE_ARTIFICIAL_DELAY as a scripting define symbol to your project's Player settings.

So if you add that to your Player settings, you'll see the host add some artificial delay to the completion of the load request. The host still sends the initial load message to clients right away, so those would get loaded instantaneously, but it'll wait that artificial delay time to simulate latency.

Yes, it's still working. Inside of DynamicPrefabLoadingUtilities.cs, it's stated:

    /// Artificial delay to the loading of a network prefab is disabled by default. To enable it, make sure to add
    /// ENABLE_ARTIFICIAL_DELAY as a scripting define symbol to your project's Player settings.

So if you add that to your Player settings [...]

So we expect that, for the sample being functional, users have to stumble across this little comment in a random script file?

1. I don't think this is something the average user will find easily, but 100% of those who don't will see the latency slider in the UI, try it, and say "this sample is broken, Unity sucks", which lowers "trust".
2. Why is this feature hidden behind a scripting define symbol? Couldn't it be on by default? If users don't want artificial latency, they just have to set the slider to "0".

P.S: I know this is unrelated to this specific PR and I'm not asking to address this now. I'd like to agree on what we want to do about this. I'll open a Slack thread to discuss this.