Yes, it is, and you can run microG on any Android version without OpenGApps. With microG you still rely on non-free services (Google Play Services, for example); just via open source frameworks. You also, possibly, breach Google's EULA.
I do use it. The notifications aren't flawless, since any application using firebase isn't going to work (the old GCM still works for now). Things may break any day. I suppose its akin to being on your own whilst running a Hackintosh except that the software is FOSS.
A reasonable alternative is a model where you pay for the software and get a license to run it. For that, you can opt for Sailfish which is almost completely FOSS. Heck, you can run community versions of Sailfish as well but without Alien Dalvik (Android compatibility layer). But I wouldn't depend on such too much.
Other alternatives are the Ubuntu Touch/Phone project by UBPorts, and the KDE people are also still alive and kicking. There's postmarketOS, based on Alpine, giving older smartphones a second life.
And you can also opt to use a dumbphone plus a mobile device. You don't need to have a smartphone.
The author talks about lineage, but not micro g, which does sound designed to solve his exact issue.
"Android experience relies heavily on Google's Play Services. The microG project creates an alternative to installing Gapps, which install and execute closed-source blobs on our phones. MicroG however requires a patch called "signature spoofing"..."
