My data

You can try this article:

Recover deleted text messages on your iPhone or iPad - Apple Support

If that doesn't work you can try restoring from a backup, if you had one.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Just a little more clarification may be needed. Even with the setting to keep messages forever, they can still be deleted manually. In that case, you may be able to recover those messages within 30 days by following this support article.

Recover deleted messages on iPhone - Apple Support

Do you have any other devices that uses the same Apple ID? If they were deleted on the other device and you use messages in iCloud, then they would also be deleted from your device.

Another thing to consider is iPhone storage space at Settings > General > iPhone Storage. If the space is maxed out, the phone will not stop receiving messages because it is full, but may need to delete some space to make room. There is no documentation on this, but it would have to refuse to receive any messages or clear on its own to continue to operate, and I would suspect it would clear on its own. You would usually have a warning about the storage space before this drastic action would be taken.