Occasionally there are - the Verizon "exclusive" version of the Moto Maxx, known as the Droid Turbo, was able to get the bootloader unlocked in the past year using the app
SunShine (by XDA's
jcase) that made use of a security flaw. SunShine only worked if you didn't have some of the newer updates installed that fixed the flaw it used. Although those who did install the updates got very lucky, as another exploit that was found gave temporary root privileges, and allowed SunShine to work on those phones too.
If your phone has an Android version with security fixes of April 2017 or earlier, you could see what SunShine's checks report to see if it might work on your phone. If it thinks it will work though it costs $25 to actually have it do its thing. I'm sure if it the checks pass and it thinks it will work, then doesn't, they will look into getting it to work, or possibly give a refund.