End of content confirmed.
Announced in 2018, but released the following year and celebrating its fourth anniversary on September 25, 2023, concerning has since come forth.
In a message from the development team of Mario Kart Tour, it has been revealed that after October 4, 2023 when the Battle Tour is set to get underway, there will be no longer be any new content released for the game in any capacity. This means no new drivers, no new karts, no new gliders and no new tracks. Instead, what we are to receive going forward, are repeat runs of previous tours, consisting of previously released content they were associated with.
The message from the development team in its full entirety can be seen here:

Source: Nintendo