If it's a BC break then you should deprecate the behaviour for 4.1 and then do it for 5.0.

As written in the referenced ticket you could deprecate the ability for a string to be handled as a callable (you can take a look at the 3.4 branch were we followed this approach for another option). However, the behaviour must not be changed here as that would be a BC break (in your own code you can use an anonymous function which just returns the string as a workaround).

Is it too early to talk about a release 5.0?
And yes.
You suggest marking the use of handler as deprecated and at same time we have only one way to solve this bug, is to use a handler.
Are you seriously?

I certainly agree that a violation of BC is bad.
I don't mind marking this method as deprecated.
But i don't want to use the deprecated method to fix not my bug.

We know that Symfony 5.0 will be released in November 2019. And if we deprecate something in 4.1 to 4.4, we also know that we will remove this feature in 5.0.

I think we are talking about different things when talking about the deprecation. I do not want to deprecate the ability to use a callable entirely. What we should deprecate IMO is the possibility to pass a callable as a string as this can be ambiguous as you said. So in the future, if you want to make use of a callable, you would need to use an anonymous function.

@xabbuh apparently i did not understand you.
You propose to mark use of handlers as a scalar value as deprecate, but to leave it possible to use anonymous function?

Exactly, you may want to take a look at #18020 where we did something similar in the past.