lager_file_backend config simply refers to the 'lager_file_backend' section of your configuration file (boss.config)
I don't know why it is being deprecated, sorry.
I had trouble finding release notes detailing the differences. I figured out how to update by looking at the example here: https://github.com/basho/lager
Here's what the old style looks like:
{lager, [
{handlers, [
{lager_console_backend, info},
{lager_file_backend, [
{"/var/log/foo/boss_error.log", error, 10485760, "$D0", 5},
{"/var/log/foo/boss_console.log", info, 10485760, "$D0", 5}
]}
]}
]},
Here's what the new style looks like:
{lager, [
{handlers, [
{lager_console_backend, info},
{lager_file_backend, [{file, "/var/log/foo/boss_error.log"}, {level, error}]},
{lager_file_backend, [{file, "/var/log/foo/boss_console.log"}, {level, info}]}
]}
]},