If we don't call this then rclone or any of its dependencies won't get properly logged if they are usinglog.Print orlog.Fatal.

From the docs

Setting a logger as the default with

slog.SetDefault(logger)

will cause the top-level functions like Info to use it. SetDefault also updates
the default logger used by the log package, so that existing applications that
use log.Printf and related functions will send log records to the logger's
handler without needing to be rewritten.

I do understand, however, we are using rclone as a library and overriding the default slog logger will mess with all our logging and causes the issues mentioned in#8907.
How about we are setting this handler the default from the rclone application? I can add a function tofs...

Just for protocol, using adefault logging facility in a library is bad style and should be avoided. Add a logger interface and allow overriding the logger in the library and then set that logger from the application is a much better pattern as it allows the application to choose how to log stuff.

I don't see an equivalent for this line in your patch? Unless the Default logger is Notice level which I don't see in the docs?

Well this line overrides the log-bridge of slog! This should not be done within the library but rather in the application so others can use the library without their logging being changed...