Why does Wikipedia state that the Solar System has one unconfirmed planet?
That was a rather recent (16 January 2021) change to that wiki page made by a user named Mariofeds, who does not have a wikipedia user page and who has just one contribution to wikipedia.
It may well have been vandalism.
The associated talk page explicitly excludes adding the hypothetical Planet Nine as an unconfirmed planet. In line with this discussion, the number of planets listed for the solar system had remained as eight confirmed, none unconfirmed for a long time.