Or did people get bored of that?

Armguard and LS nerfs made it less OP. It's still okay, but it's not free ELO anymore.

All I know is that I've seen it once against a Kassadin and he[lee] did excellent

Baby if you want it to be a thing we can make it a thing for you

Yes.
Played against one as Akali once.
Counterpick? Lolno

Everyone building warmogs not to mention armguard just made assassin lee less viable, plus what everyone else has been saying. Still do-able tho

AD casters died off a long time ago when sunfire rush > every AD caster ever. Lee's base damages and scaling aren't high enough to justify just rushing sunfire mogs then a black cleaver.

Cloth 5 viable = Lee mid not viable.