This would be an E_DEPRECATED
error and that may be coming from its usage within - I think - the Swift mailer library.
However, if you're seeing this logged or it is blocking execution then it may indicate that you're running with debug mode enabled or you have a custom error reporting level set.
This is not recommended for sites in production and that is what should be addressed for now.
Starting with XF 2.2 we are using a newer version of the library that does not attempt to call get_magic_quotes_runtime
.