What should I do if the iphone6 SMS is suddenly deleted automatically at the end of the month?

Settings-Information Options-Keep Information (January/Year/Permanent Retention). Choose the time to keep information according to your own needs. You can set it so that the information will not be deleted automatically.

Delete: Delete unnecessary files from the system directory list to make room for other operations. The "complete" deletion in the computer is mostly to delete it from the list and can be overwritten by other files. So don't move after deleting it by mistake, there is still hope for recovery! For the sake of safety, users had better get into the habit of backing up their own input text or other files on floppy disks, just in case.