Tested in 5.4.5. Same issue.

You can not switch the session handler based on an env variable as it can not be changed at runtime, it requires a whole cache clear. You would need to switch to a parameter instead of env var.

There are some discussion about static vs runtime env vars here: #40794

Closing here as explained by @alexander-schranz. Thank you for understanding.

@alexander-schranz thanks for your reply. The whole purpose of setting the session handler via an env var is to keep this setting variable and isolated from the version control software. If I set the session handler using a parameter in framework.yaml I would have to keep the entire file away from my control version system. I need to keep framework.yaml tracked by git with the ability to switch between session handlers without messing with deploys. Is that possible? Thanks again.

It is not possible as env variables are designed to be used only for configurations which do not require a cache clear. For using env variables for configs which do require a cache clear the linked issue is about it (#40794).

You could still use the parameters.yml/parameters.yml.dist like it was in symfony 3/2: see comment here: #40794 (comment)

See also the linked presentation here where nicolas did talk about runtime parameters and config parameters and so on: #40794 (comment)

As you see the last comment of the linked issue is "Help wanted", so symfony is open here for a pull request to introduce a special env resolver to achieve this things e.g.: #40794

Until you need as said go with something else, a parameters.yml or some hack in your Kernel.php to set a kernel parameter by $_SERVER/$_ENV variable to achieve what you want. Or use the .php config instead of .yml and use $_SERVER or $_ENV to achieve what you want.

@alexander-schranz thank you very much. I will be back on this if I find a proper solution.