Percy's Comfort Kit is a Minecraft datapack that contains several methods to sit. There is an immersive method to turn stairs into chairs using carpets, and a trigger that let's the player sit anywhere they like.
Using carpets
Drop any carpet item onto a stair block cutout to make it a chair. Make sure the stair is upright.
This summons an interaction entity the player can sit on with an item display entity showing the dropped carpet.
To remove, punch the carpet or remove the stair block. The carpet will be dropped.
/trigger PCKSit
Trigger this to summon an invisible entity that the player sits on. The entity disappears when the player gets back up.
Drop any carpet item onto this entity to summon the same interaction entity with the carpet, as described above. This entity persists and isn’t removed when the player stands up.