Obito's Clothes Affected by Kamui?
Does that mean that a person holdings with him or a backpack he's wearing is also intangible? Yes, anything touching the individual who uses the Kamui is also affected by the justu
Kamui

Right Eye
Obito's right eye uses a short-range version of Kamui, which can only transport targets in close proximity to the user; physical contact with the target is seemingly required
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This intangibility can be extended to anything the user is touching at the moment of activation

Anything that is touching the caster becomes intangible, including clothes.
In Naruto Manga Chapter 684, we see Obito instructing Naruto and Sakura to keep their hands on his person so they may teleport alongside Obito