I accidentally deleted my sent mailbox in apple mail (iCloud). Can't restore.

Delete the mail application account Remove email accounts in Mail on Mac - Apple Support, and once again configure the account Add email accounts in Mail on Mac - Apple Support ( make sure you have all account passwords and mail address / Apple ID ) .

The inbuilt mail application folders sent , inbox ... will once again reappear and all mails from web accounts will sync in respective mail folders .

It all depends upon your internet service provider , if years back sent mails are not on server ( for POP accounts ) , they are erased from server and won't be synced , generally mails are kept for 5 to 7 years .