Great for families or squads that live and game together.

I guess this is more for SteamDeck since it is already easy to copy files on a LAN lol

Will it happily transfer my Steam version of Fallout 4, the one with over 300,000 mods installed, without causing the Red Phone on Gabe's desk to start ringing?

Celcius:

Will it happily transfer my Steam version of Fallout 4, the one with over 300,000 mods installed, without causing the Red Phone on Gabe's desk to start ringing?

no

Without DRM (if sold like that of course) it is even more easy...

I've taken a different approach to this: Have your Steam library stored on a network-mounted server. That way, you don't even have to download the whole game - you just play it over the network. This is also nice since it allows you to not have to re-download content across multiple devices, if you're stingy about storage. I don't really recommend this for large/newer games or over wifi, but even then, all you have to do is move/copy the game folder to a local drive, which is bound to be faster than re-downloading over the internet. I mostly just do this with games that are either small (like indie games) or with games that were designed to run off a HDD or disc, since even gigabit ethernet is faster than what those would normally provide.

It really needs to work like downloads where you can set a max speed cap so no one swamps the lan. Would prefer the cap to an "on idle" only setting as I don't mind my machine doing some data transfer in the background, I won't notice. Also "on-idle" doesn't help if PC1 is transfering from PC2 and swamping the LAN while PC3 is attempting to play an online game - PC3's user is going to rage.

Very sensible enhancement from Steam. Right now both me and the sons PC both have to download all game updates over network, and he is only here every other weekend so when his Steam client starts downloading an update, my PC is very unresponsive as the LAN is swamped by his 20MB/s updates. I guess I will have to use the "Friends" option here then. New EA app needs to do the same thing - I'm still stuck with this for a lot of EA games eg Battlefield, Apex... and same problem happens with their massive updates!