I was able to work around this issue with NSInvocation in Objective-C:
WKWebViewConfiguration *config = [[WKWebViewConfiguration alloc] init];
SEL selector = @selector(setWritingToolsBehavior:);
NSMethodSignature *sig = [config methodSignatureForSelector:selector];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:sig];
[invocation setSelector:selector];
[invocation setTarget:config];
NSInteger arg = (NSInteger)UIWritingToolsBehaviorNone;
[invocation setArgument:&arg atIndex:2];
[invocation invoke];
This works to set the writingToolsBehavior to none, and I don't get the Writing Tools option in my popup when selecting text in my WKWebView.