"Mainline kernels" also gets used to refer to kernels built from Linus's repo in general, as a way to distinguish them from kernels built from the OS vendor's / hardware vendor's branch.
For example when people talk about phones or tablets having mainline support, they mean that Linus's tree has all the drivers etc for that hardware and using the hardware vendor's arbitrary kernel drop isn't needed. They don't necessarily mean that the support is only in master and not in a stable branch. Eg https://mainline.space/https://not.mainline.space/
The thing is you don't want Linus's kernels unless you are a kernel developer or testing something. Greg KH maintains the stable kernels. You want his branch.
For example when people talk about phones or tablets having mainline support, they mean that Linus's tree has all the drivers etc for that hardware and using the hardware vendor's arbitrary kernel drop isn't needed. They don't necessarily mean that the support is only in master and not in a stable branch. Eg https://mainline.space/ https://not.mainline.space/