sitting block

[fickle knot]

Is there any block component that the player can sit on?

[vestal canopy]

No. Use the script api to summon a custom entity that can be sat on and the using /ride or the script api version to make the player sit. Then when you dismount kill the entity.

Any feature that is not vanilla or even not super common requires that you code the feature yourself.

[shy totem]

Now I could use the inputpermission command and an animation to simulate the player sitting, and the state would only be removed with script detecting the player crouching or jumping.

Just as an option, if you don't want to use an auxiliary entity

[south pier]

i made this a while back when i used to make addons if it helps