I don’t know if it’s really a bug, but as stated in https://stackoverflow.com/q/48909964/6028808, when a handler for the logout is specified using a lowercase for second part of the name, i.e. Logoutlistener, symfony tries to find an unexisting file named with a 2.

I saw, after having found the solution, that declaring with lowercase is deprecated in 3.4.x and throws an error in 4.

The purpose of this issue is to, maybe, improve the error message, the error was on my side, as I had the typo in my code but was hard to find.
Furthermore, as it is deprecated but still in use in 3.4, I think it should « compile » without error, except the deprecate warning, so I think the error is not attended, that’s why I submit it as a bug, but feel free to close, if you don’t think it is.