Message:
The alias option of the form.type_extension tag is deprecated since version 2.8 and will be removed in 3.0. Use the extended_type option instead.
409 is where we're currently discussing the 3.0 deprecations, looks like this one's already been mentioned in there and someone said they're working on a PR to clean up a lot of the known remaining ones.
Please fix following deprecation:
Message: The alias option of the form.type_extension tag is deprecated since version 2.8 and will be removed in 3.0. Use the extended_type option instead.
File: https://github.com/braincrafted/bootstrap-bundle/blob/develop/Resources/config/services/form.xml