Upgrading to 17.0.1.0 (from 17.0.0.
appeared to have remembered the setting to
Indent with Tabs (in Tools > Options > All languages > Formatting). The option was still checked in the dialog, but editing a C++ file immediately after the upgrade revealed that spaces, not tabs, were being inserted into the file.
I had to toggle the option off, Apply, and back on, Apply in order to restore the Indent with Tabs behaviour. I remember that this was a problem for previous upgrades as well (during the v17 beta).