All Japanese carriers offer free SIM Lock unlocking service 100 days after purchase or 100 days after a previous SIM Lock unlock of the same contract mobile number, which ever comes first. Day 1 iPhone XS users are just past the 100 day mark, I successfully unlocked my Docomo iPhone XS SIM today. You can do this at your local carrier store for ¥3,000 but it’s free when you do it online via My docomo, My SoftBank, My au. Have your iPhone IMEI number ready: go to Settings > General > About and scroll down to the IMEI number to copy it. Be sure to remove any spaces between number groups so it is one unbroken number string.
I don’t know about SoftBank or au. Docomo is a little sneaky because they hide the SIM Lock removal option in the iOS My docomo app. You can only access it from My docomo web site. The page and procedure is Japanese only but there is an outline of the procedure and conditions in English. Here are screenshots of the process in case you get stuck.