Yes, Steam client updates can be downloaded manually via HTTP, if the names of the update files are known.

In my case, Steam seems to be unable to handle the poor Internet connection. When the network gets interrupted briefly, Steam just issues an error and exits; if I start it again, it can't resume the process and everything have to be downloaded again. So I'd like to use a robust download utility that supports pause and resume (i.e. HTTP Range Request).

The files it failed to download can be found in bootstrap_log.txt. (It is located under the directory ~/.local/share/Steam/logs on Linux). For example, this is one of the entries in the log:

Package file tenfoot_dicts_all.zip.c74a3a9beb77a280cfd8761b901a80ed0f6a3173 missing or incorrect size

To apply the updates manually:

• Using your favourite download utility, download http://media.steampowered.com/client/ + package file name. In this case, it's http://media.steampowered.com/client/tenfoot_dicts_all.zip.c74a3a9beb77a280cfd8761b901a80ed0f6a3173.

• Put the files you've downloaded where Steam Client would. On Linux it is ~/.local/share/Steam/package/

• Start Steam.

And now Steam should be extracting the updates right away, without downloading them again.

Credit: aiusepsi on the r/Steam community on Reddit. Link to post

There is no generic setting in Steam that prevents updates

There is no generic setting within Steam that prevents game updates from automatically being downloaded. The only way to prevent auto-updating is to run Steam in offline mode - which will prevent you from playing Steam games online.

It is not possible to prevent Steam client updates from being downloaded while the client is in 'Offline' mode.

There is a per game setting, though...

There is a per-game setting which allows you to disable updates for individual games, which is accessible by right clicking on a game in your library, and selecting properties and choosing "Do not automatically update this game":

It is worth noting that if you're playing a game on Steam, all Steam downloads are automatically paused unless you select the download option "Allow Downloads During Gameplay" (which is available through the Steam menu > Settings > Downloads category, at the bottom), so you shouldn't experience Steam downloading files while playing Steam games. Note that this setting will not prevent Steam from downloading updates when you're playing a non-Steam game.

Now there is a generic Downloads Restriction setting, too!

Under Steam > Settings > Downloads, there is now an option available that allows you to specify a specific time frame to download updates. If you know you're going to be using mobile data for a specific time frame, you could use this functionality as a workaround to ensure that updates are not downloaded during that specific time frame: