My sim card with my old number has been deactivated. A new user uses my old number. Does the user have access to my old chats if he or she chooses Yes to back up old chats or storage?
Whatsapp backups are either stored locally (on you device), on your own Google Drive account if you are using Android or on iCloud if you're using an iPhone.
In order to restore a backup you must be using the same phone number and the same Google/Apple login, so you can rest assured that the new owner of your old number does not have access to your old backups.
If you want to be doubly sure and you use Android, you can log on to Google drive and find your Backups folder. In there you should see a file named "WhatsApp YourOldNumber backup", you can delete this if you want.
I don't own an iPhone but I imagine the iCloud backup would be stored in a similar fashion.