That line won't help much probably, because it will print error, warning and parse error messages then, which for old PHP code on i.e. 5.6 can still be a lot (i.e. variable x not defined but used or stuff like that).
What you need to change is the logging of PHP errors and warnings and notices into the error_log.
Try changing log_errors to Off in your php.ini:
; Besides displaying errors, PHP can also log errors to locations such as a
; server-specific log, STDERR, or a location specified by the error_log
; directive found below. While errors should not be displayed on productions
; servers they should still be monitored and logging is a great way to do that.
; Default Value: Off
; Development Value: On
; Production Value: On
; http://php.net/log-errors
log_errors = Off ; Original value for me was On
I don't recommend moving to 7.x yet, I'd expect a lot of old code that has problems with 7.x.
Also this would not solve the logging problem.
I don't know if php.ini is preserved or not when switching versions in CWP.