This issue now affects the stable version.
To work around the issue you have to set Interface -> Start Up Location to Library. With this setting, the new client will respect -silent.
With any other setting, it will ignore -silent, and also revert the client to "Large mode" on startup, if you had previous set it to "Small mode".
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Please describe your issue in as much detail as possible:
Steam still shows "looking for updates" dialogue and login spinner when starting steam using
steam -silentSteps for reproducing this issue:
steam -silent