Why has support for a large number of phones been stopped?

[AnotherElk]

And the technical unability or unfeasability to get the phones in question to run newer Android versions.

Or, in the words of the weekly developments updates you can follow at Topics tagged development-updates …

@Manoj

Why /e/OS stops support for an OS

Google has stopped security patches. LineageOS has also dropping support. For most of the older devices vendors would also have stopped vendor patches.We would not be able to continue support as it would require backporting security patches which will require dedicated resources and is not a viable long term solution. Users are advised to upgrade to newer OS and builds where available.