UNET is all about the low level api (LLAPI). That gets updates all the time - it just got multithreading a few weeks ago, which is huge. The HLAPI was meant as an example on how to use the LLAPI. Lots of people used that too. The HLAPI developer left Unity. Hence HLAPI was somewhat abandoned. UNET is still going strong though - LLAPI is what matters. And now we have some HLAPI fixes too anyway.