What changed
Plasma: the exit only works if the data was published.
The paper's thesis was a tree of chains, with the parent as the court. The hard part was proving you could leave when the operator withheld data.
Original thesis
Keep execution on a child chain. Post commitments to a parent. Let users exit if the operator cheats, using a fraud proof and a challenge window.
What changed
If the operator withholds the data, users may be unable to construct the exit. Many Plasma variants were not deployed as specified. Later Ethereum scaling work moved toward rollups that publish data, or prove it, so the exit can be built.
What remains useful
The paper is still the citation for interactive dispute and the data-availability problem. It is not a description of a current network.