Can I use javascript urls in react16.9?

Yes, you can use javascript: URLs but it will log a warning in the console. Because URLs starting with javascript: are dangerous by including unsanitized output in a tag like <a href> and create a security hole.

const companyProfile = {
  website: "javascript: alert('Your website is hacked')",
};
// It will log a warning
<a href={companyProfile.website}>More details</a>;

Remember that the future versions will throw an error for javascript URLs.