I've since forgotten if this was ever the case and/or changed but at some point in the past I believe conjurers lost constitution if the familiar died. The only obvious result for a familiar death is damage to the conjurer equivalent to a backstab depending on the conjurer's level. But there is no echo regarding vitality loss. Is this still the case that they lose 1/3rd con if their familiar is killed?

This is still the case - you lose 1/3rd of a con pt as if in a normal death when you familiar dies, and you take a bunch of damage which could in turn kill you causing you the equivalent of 2 deaths. Good incentive to guard that precious treasure.

I died twice to mobs, still at 20 con.

Familiar died instantly to a thief backstab, still at 20 con.

I started scratching my head, my con should be 19 if that's the case. Plus the familiar helpfile should indicate vitality loss instead of just 'great pain' but that's been the case for years.

There are some quests you do which restore partial con. If I had to guess, I'd wonder if maybe you got one of those in somewhere around there.