/Library/Preferences/com.apple.MobileSMS.plist  is shown in this slide. If you are missing message from the device, this is a great place to start.

KeepMessageForDays 0 = Forever, 365 = 1 year, 30 = 30 days – iOS 14+

KeepMessageForDays NULL Value = Forever, 365 = 1 year, 30 = 30 days  - iOS13 and below

com.apple.MobileSMS.plist com.apple.MobileSMS.plist 88 v Root com.apple.MobileSMS.plist > No Selection kHasSetupHashtaglmages DidCheckForDuplicateChats _CK_clearTextlnputContextldentifierKey ReadReceiptSettingsConfirmed KeepMessagesVersionlD KeepMessageForDays MMSRelaYEnabled CKMigratedAutoSpamReports26375262 CKTextlnputldentifiersMigrated PendingCleared DidMarkGroupph0tosAsUnpurgeable Type Dictionary Boolean Number Boolean Boolean Number Boolean Boolean Boolean Boolean Boolean Value (30 items) 3 2