Muramana is still and has been a moderately viable item for laning Kha'xiz, depending on the lane match up, desired power spike, and intended playstyle. For one-shotting enemies in mid game, muramana is the best option on kha'zix and always has been (again, laner xiz is assumed). What it does for you is:
- Provides mana (obviously); the purpose of this mana is to spam W for sustain, wave clear and poke in lane.
- Provides a MASSIVE spike at 2-3 items (muramana+ghostblade/ muramana+ghostblade+lw)
- Provides an enormous source of damage on auto attack (due to interaction of his passive with the on-hit damage) and Q
- Provides a large amount of raw AD for scaling abilities
The reason it's fallen off in popularity is that most of those things have been nerfed. His W is no longer as effective as poke/healing as it was when laner zix was all the rage (this is the biggest single reason it's not as popular), and additionally, his W no longer procs the muramana, making its poke less effective in two ways. His scalings, especially E scaling, are not as good as they were at the time. Still, in some matchups muramana is amazing, and in terms of raw damage output, it's got the most of anything you can get for him.