LineageOS devices are subject to desupport at any time, when the maintainer(s) makes the decision to retire it (and no volunteers appear to replace the maintainer).
With the sunset of 3g, many devices that were not capable of 4g were retired.
That being said, Lineage still does support several Pixels that Graphene has desupported. In fact, I am typing this on a Pixel 3a XL which is in this category.
The original Pixel, Pixel 2 and 2XL are still supported by LineageOS:
Edit: This extended support does come at some risk. The chipsets used in Android can sometimes have showstopper bugs. Sometimes, these cannot be patched, although maintainers will go to great lengths to find solutions. The "firmware age" is reflected in the "vendor security patch" date in Lineage - the older, the more dangerous.
