On LineageOS, there is no "stable" build, but despite the naming, "nightly" builds are considered stable.
There was a discussion on Reddit, Why are all downloads of type "nightly"? where it was revealed that that is just how LineageOS went with the naming since it is always built from the latest commit automatically (i.e. "nightly"). An official answer by a mod of the subreddit said,
LineageOS is a "rolling release" software. It means that new builds are continuosly produced (every week for each device since it takes a long time to get through all the devices builds without abusing the hardware) and there's no "stable" / "recommend" release.
All our official builds are meant to guarantee you the hardware will be working (from 15.1 onwards this is enforced, but most 14.1 builds should be fine too).
Another reply by user 'goosnarrggh' also explained the distinction from CyanogenMod's nightlies,
As you correctly inferred, the nightlies are produced automatically using the most recent commit on the currently-supported branch of development for each component of the OS. That's all that officially exists. Despite the word "nightly", however, each device receives a build once per week (all things being equal) due to limited build hardware resources.
Every device listed with downloads from LineageOS's website does have an active maintainer, and those devices do provide a minimally complete functionality
as spelled out in an informal process that will be formalized soon through the project's charter. However, bugs can and do crop up occasionally from one build to the next; sometimes they're device-specific and sometimes (but more rarely) they may be project-wide.
In LineageOS's predecessor, CyanogenMod, there used to be a distinction drawn between the bleeding edge of development, "nightlies" and feature-frozen builds which had been manually judged to be of adequate stability, labeled as "snapshots". The snapshot builds would have been proposed for points in the OS's development in which the feature set was considered reasonably functional, and then each device which was demonstrated to be in proper working order would receive a build based on that snapshot. LineageOS has not continued this practice; at least in part, this is due to the demands that workflow would have imposed on the project's volunteer maintainers.
This is unlike CyanogenMod which has "snapshots" for stable builds, where "nightly" can be considered unstable.