Does "Data Saver" also apply to metered Wi-Fi? The section Use Android Nougat’s "Data Saver" (Android 7.0+) under this site How to Restrict Background Data for Metered Wi-Fi Networks on Android mentions:


  Android 7.0 Nougat introduced a much more granular way to take the reins on your mobile data with a new feature called Data Saver.
  
  Basically, this allows you to limit background data used by apps, but whitelist anything that want to have unrestricted access. This means background data is disabled for every app by default, then you can pick and choose where to grant unlimited access
  
  And that’s all there is to it. It’s worth keeping in mind that this only applies to mobile data-all apps will remain unrestricted while on Wi-Fi.


My question is whether this Data Saver option applies to Wi-Fi that you set to metered, meaning metered WLAN?
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.