PoS hasn’t had much impact. The leveling out there is more like related to fewer transactions. Proto dank sharding, which may ship in 2023, should have a bigger impact on gas fees.

If you wanted to see the effects of the PoS switch (3 months ago), you'd have to choose a shorter timescale, like 6 months. It's been going down slightly, but not drastically, and only in proportion to the decrease in daily transaction count (as simpsond mentioned).