Does "Data Saver" also apply to metered Wi-Fi?
Yes, Data Saver option applies to metered Wi-Fi. As quoted here:


  From Android 7.0 (API level 24), users can enable Data Saver on a device-wide basis in order to optimize their device's data usage, and use less data.
  ...
  When a user enables Data Saver in Settings and the device is on a metered network, the system blocks background data usage and signals apps to use less data in the foreground wherever possible.


Android achieves this through iptables and I can confirm (tested on Pie) that REJECT rules are added for wlan0 interface when both conditions meet i.e. Data Saver and Metered WiFi.