"Turn on Wi-Fi automatically" will be enabled by default on all Android 9.0 Pie running phones

Pixel 2 XL on Oreo has the Turn on Wi-Fi automatically feature
Not all Android phones running Oreo have the feature since it is up to each manufacturer to decide whether or not to enable it. If your handset does have the "Turn on Wi-Fi automatically" feature, you can turn it on by going to Settings > Network & Internet > Wi-Fi > Wi-Fi Preferences > "Turn on Wi-Fi automatically." Wi-Fi scanning will need to be enabled, which can be set at Settings > Location > Scanning.

Commit code from AOSP reveals that the Turn on Wi-Fi automatically feature will be enabled by default on Android 9.0
source: XDA

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