This was a little confusing at first, but I think I figured it out , but after everything was said and done, Fallout 4 is no longer asking to update, and I was able to successfully play my modded game all last night without any problems and seemingly subverting the update.
So, I thought I would share my method for anyone else who might have also been confused. I wasn't sure if you were supposed to change the manifest ID for all of the Depots, or just the last one until I went to the SteamDB page and clicked around a bit.
Here's what I did:
1) Yesterday I set the appmanifest_377160.acf file to Read Only (and changed Updates frequency to Only when launching) as directed in the OP.
Today, Fallout 4 showed [ Update Required ].
2) Since I don't want the update (at least, not now) - using the Depot ID and Manifest ID for -each- row from my .acf file, I changed the Manifest ID to the new one.
(If you click on the Manifest ID link on the right of your matching Depot ID, you will see:
Example for Depot ID 377161 from the SteamDB page: https://steamdb.info/app/377160/depots/?branch=public (as directed)
Depots
ID
Configuration
Manifest ID
Size
DL.
228984
Windows Shared InstallDepot from 228980 VC 2012 Redist
No size
228990
Windows Shared InstallDepot from 228980 DirectX Jun 2010 Redist
No size
377161
Fallout 4 content_a
7332110922360867314
5.67 GiB
5.21 GiB
3) So for the first Depot 377161: the link on the left will take you to a page that has the following on the bottom:
Manifest ID changed – 7497069378349273908 › 7332110922360867314
Aha, now I could see that my (original) file matched the OLD ID: 7497069378349273908
"InstalledDepots"
{
"377161"
{
"manifest" "7497069378349273908"
"size" "6092666130"
}
4) So I changed the "manifest" in the file (with Notepad) to the NEW ID from the website: 7332110922360867314
It now reads:
{
"377161"
{
"manifest" "7332110922360867314"
"size" "6092666130"
}
5) I then did the same steps for each of the remaining Depots and Manifest IDs. I only have VANILLA (NO DLC), so there were only 4 matching Depot Manifest IDs to change.
6) I exited Steam, Unset the Read Only for the appmanifest_377160.acf file, and then SAVED the changes in Notepad.
7) After saving -- I RESET the Read Only on appmanifest_377160.acf --- otherwise, Steam might try and get the 'real' update.
Upon launching Steam, Fallout 4 now shows [ Play ] -- so I believe Steam thinks FO4 is current, and I'm set.
Hope this helps someone!