How to reduce Safari/WebKit logging verbosity?
While the accepted answer probably represents the right thing to do, there’s another way to narrowly answer the question, i.e. how to reduce the logging verbosity of the WebKit subsystem (permanently):

sudo log config --subsystem 'com.apple.WebKit' --mode "level: error"


Now, only error-level messages should be logged.  (Caveat:  I haven't tested it.)

But I won't recommend this;  rather just filter for the messages you need.