Well, I needed to use a computer. No longer works from the privileged Terminal app, not even with
$ adb shell settings list global lists
wifi_sleep_policy=2 on my FP2’s LOS for microG 15.1 installation (upgraded from 14.1) and my Nexus 4’s official Lineage OS 15.1 (installed from scratch, so I’m sure it’s not a residual config).
I successfully changed it to 0 with
$ adb shell settings put global wifi_sleep_policy 0 on my Nexus 4. I don’t know yet if that effectively changes the wifi behavior, though. It seems to be completely deprecated in Android 9, so I won’t bet a cent for it in Android 8…
(BTW, the command line code is here)