You will now receive a processId along with total (backward compatible) in response schema when you pass the parameter "all": true in Delete a contact from a list endpoint.
This is because a process will be created which will handle this task asynchronously.
You can check the status of the process using the processId in the get process API.

You will now get the Id of the reseller child as a number along with authKey (backward compatible) in response when creating a reseller child.

Total number of non-delivered campaigns for a particular campaign id or all campaigns can now be fetched if it exists using returnBounce field.

from and tags have been added in the response schema as they are relevant fields to have for reporting purposes. These will appear in the response only if they are present in the client’s DB.
Api Route: https://developers.sendinblue.com/reference#gettransacemailslist

SMS number can now be unset if the contact has a valid EMAIL address. Also, error for duplicate SMS will now be thrown if the SMS is updated to a number which belongs to an already existing contact.

SMS number passed in Contact create/update api will only be accepted if passed with proper country code & number combination.
Some examples of accepted numbers

startDate and endDate filters will now be available while getting contact specific campaign stats. These will narrow down the campaign stats for a contact on the basis of input dates passed as query parameters
Route: https://developers.sendinblue.com/reference#getcontactstats

API V3 Route: https://developers.sendinblue.com/reference#importcontacts-1.

You will now be able to upload an image to your account's image gallery by using absolute url of the image. Maximum allowed size of the image is 2MB. Allowed formats are: jpeg, jpg, png, bmp, gif.

The contactFilter attribute is now marked as Deprecated and will be discontinued after Jan 1, 2021.