6th season and we still lose LP when playing outnumbered.

You lose LP when you lose a match. You're likely to lose a match when you're down a player.

The two things are not directly linked. You're not being punished for it, and you're not having anything happen specifically to you. Ranked is a game of averages, and over time it will all balance itself out.

You can see it as a problem and suggest however many solutions you want, but it's ultimately up to whether Riot thinks it's a problem or not, and whether it's worth fixing. It doesn't simply get fixed because you want it fixed.

From what I can tell they would rather fix the problem of people leaving matches, rather than fix your own personal LP loss. I'm not saying they can do that, but that's what it looks like.