As title says The Modifier Proficiency - Self does not work with weapons only armor. In previous editions there have been magic items that give proficiency to that weapon and that weapon only while wearing it, like the Elven Chain. Since the modifier does exist and does work i expect it to be an error for the weapons to not, but would like a confirmation at the least please. A great example would be sentient weapons giving proficiency to the user while they are wielding or attuned to them, or even the really long time ago limited universal weapon versions that anyone could use (at lower damage, kinda like training equipment). Any information would be greatly appreciated thank you.

Correct, the modifier only works for armor not weapons, because there weren't 5e official content magic weapons that granted proficiency with the weapon.

I think (hope!) it's on the list of features for the upcoming general homebrew tools.