Household Sims and NPCs wear shoes in the house although the No Shoes rule is active. They are always shoes haven’t chosen anywhere.
Slippers work well.
Cause & Solutions
Climbing Gear. If you’re Sim is equipped with climbing gear, he will also wear shoes as they are part of the gear. The Sim will also go through the animation but still wear the same shoes he had before. Details can be found here: My Sim is wearing gloves/shoes everywhere – even while taking a shower
Sims can be naughty and not follow the rule. Some Sims don’t feel like follow the rule. If that happens there will not be a shoe changing animation and the Sim will then have the option Follow Household Shoe Rules when you click on the active Sim. Proper Sims will get a negative moodlet because of this.
If you have CC Feet in your game, then this is the cause. Those need to be updated. If you see a Sim appear in your house and you can observe the animation to change to no shoes but then they just switch to a different kind of shoe, then that’s the issue. It’s probably only default-replacement feet. Feet that are in fact just socks shouldn’t give any trouble.
Remove or update the CC Feet