About old data
If I change my password and logout of all the other devices. Now the person who will be using my iPad with his account and ID, will he be able to check my iMessages, which were there before?
iPhone 16 Pro Max, iOS 18
If intending to give-away your iPad for reuse by someone else, be sure to prepare the iPad properly before doing so:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
This is essential to ensure that none of your personal/sensitive information can be accessed.