It would be nice if steam had a minimum date for steam-runtime, and if the runtime is new enough, it could be updated and/or download only in the background (to update on next start) while the steam client comes up in a timely manner. The point of the minimum date would be to simulate the current behavior when steam needs something new in the runtime at the same time as an client update.
Adding an abort to this should be a one liner to check for STEAM_RUNTIME=0 and skip the runtime update cycle.
Nothing extracted yet.
As of now the steam client/updater downloads the package of the steam runtime regardless of the user setting. It also unpacks the package ( which contains another archive :D ).
The one thing setting STEAM_RUNTIME=0 disables is unpacking the 'second' archive.
I have a small ssd for my system files and some other files, so space matters In total this clutters my drive with ~200MiB of additional files I already have installed anyway.