Now that Pluto is no longer considered a planet, which is now Scorpio's ruling planet?

Scorpio is now dually ruled by Pluto and Mars. Astrologers don't seem too concerned by the decrees about planetary status made by astronomers. Mars was considered to rule both Aries and Scorpio before the discovery of Pluto; the existence of Pluto brought a new allocation into popularity. (Given that Pluto was there before astrologers knew about it and started to describe its impact on people, it is not all that surprising that they haven't abandoned all their analysis about its influence - they just didn't know about until around 1930!)

As is the case with all dually-ruled signs (the other two are Aquarius and Pisces), some people born under this sign are more attuned to the power of the old planet, and some are more attuned to the power of the new.
Astrologers generally agree that interpreting Pluto as Scorpio's ruler is more precise and insightful for the majority of Scorpios.

https://trustedpsychicmediums.com/scorpio-star-sign/scorpios-ruling-planet/

I will add that from doing the relevant quizzes here, ruling planets include all heavenly bodies in the solar system including the moon and the sun, so as stated above it's the body not its designation which is assigned.

Here is the complete list, and you will also see Ceres has been recently added as well.

https://en.wikipedia.org/wiki/Domicile_(astrology)