How will the app behave of monolog is not used? And how does monolog fail if configurated to write there?

How will the app behave of monolog is not used?

No var/log is created

And how does monolog fail if configurated to write there?

When logging it throws a exception:

In StreamHandler.php line 140:
                                                                                                                                                                 
  The stream or file "<project>/var/log/prod-2024-10-17.log" could not be opened in append mode: Failed to open stream: Permission denied  
  The exception occurred while attempting to log: Error thrown while running command "asdasd". Message: "Command "asdasd" is not defined."                       
  Context: {"exception":{},"command":"asdasd","message":"Command \"asdasd\" is not defined."}          

I think we periodically wonder about this, and the conclusion has also been: a writeable var/log is a feature of the kernel, it doesn't relate to monolog directly.
I think it's fine for apps with non-conventional needs (readonly fs) to have to do a few more tweaks.
APP_LOG_DIR=/tmp/log is the way to go to me.

Maybe deprecate it for Symfony 8.0, remove this method, and rely only on configuration for path /var/log?

getLogDir is just passed as a parameter to the DI container.

Sure I can set APP_LOG_DIR to tmpdir, it feels hacky. Symfony Cloud (platform.sh) seems to have a hack that this folder is always writeable, it would also improve the official Symfony product

As mentioned by @nicolas-grekas, we've had this conversation a few times now. I think more and more containers are read-only nowadays. I'm fine with this change, but probably not as a bug fix for 5.4. I would be ok to merge it in 7.2 though.

updated the target branch

Thank you @shyim.