The amount of times I saw a "LinkedIn profile URL" as a required field on job applications outside of LinkedIn is concerning, to say the least.
What I do is put the link to the job listing in that field. haha, suck to be in HR if you think you're getting my profile info!
I've never seen any ATS that required Linkedin for login, but even having it as an option I've always thought of as weird.
Then again, I have see many an ATS that include Facebook and other slop sites, so welcome to another layer of jobhunting madness.
I think it’s kind of companies to tell you they don’t want applicants.