Dear Michal Rama,
Thank you for posting in Microsoft Community.
Per your description, you want to disable the Check in/Check out function in SharePoint library.
As far as I know, I'm sorry, there is no built-in feature to disable the function. Very sorry for the inconvenience.
The option Require documents to be checked out before they can be edited is only used to require check out or not check out a file when a user open/edit a file so that it can control document collaboration. It is not used for disabling Check out.
Personally, I understand your concern and I recommend you send feedback to SharePoint Feedback Community. Microsoft product developers team monitor this site and they will receive your request and put it into consideration. This is the best platform to let related team hear from you and make the products and services better for you and others.
Thanks for your cooperation and understanding. I hope that you are keeping safe and well!
Sincerely,
Tina | Microsoft Community Moderator