(All Japanese carriers are required to sell unlocked mobile phones since 2020, this post remains for reference)
All Japanese carriers offer free SIM Lock unlocking service 100 days after first purchase with contract, or 100 days after a previous SIM Lock unlock of the same contract mobile number, which ever comes first. I was already past the 100 day mark and immediately unlocked my new Docomo iPhone 11 Pro the moment I got it. One nice thing that Apple does in case you ever have to exchange your carrier unlocked iPhone for repair: they automatically provide you with an unlocked iPhone.
You can unlock your iPhone at a Docomo shop or call Docomo and ask, but it will cost ¥3,000. It’s free when you do it online via My docomo (or My SoftBank, My au if you have a different carrier). Have your carrier IMEI number ready: go to Settings > General > About and scroll down to the carrier 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, but Docomo is a little sneaky: they don’t include the SIM Lock removal option in the iOS My docomo app service menu. You can only access it from My docomo web site, way at the bottom of service options listed under ‘other service options’. The page and procedure is Japanese only but Docomo has an outline of the procedure and conditions in English. Here are screenshots of the process with captions in case you get stuck.