Yevgeny Prigozhin, head of Wagner, is reported to have been on a small plane that crashed, or (some say) was shot down by the Russian army.

I wouldn't take any of this as certain. But if Prigozhin is alive but in hiding for the long term, he won't be able to run Wagner effectively, so for most practical purposes it would be equivalent to his death.

If Prigozhin was indeed killed by that crash, it means the end of Wagner too.

This may benefit Putin by removing apparent instability, but I think it will weaken Putin's in Russia and deny him what Wagner used to contribute to the Putin forces' military strength.