Which expression is correct? “in development” or “under development”?

Either is correct, you can meet both versions used for the same context by the same source, for example:

http://www.diabetes.co.uk/research/diabetes-drugs-in-development.html

The header:

Diabetes Drugs and Medication in Development

a listing with description and a header:

The following diabetes drugs are currently under development.

and then immediately after the above:

Diabetes drug treatments currently in development

But if a product is not yet developed, the adjective to describe the status is 'underdeveloped'

As the final proof the expressions are completely interchangeable, use these Google search queries to see results using both expressions in the same situations, remember to include double quotes:

• "products in development" "products under development" Apple
• "products in development" "products under development" Microsoft
• "products in development" "products under development" Google
• "products in development" "products under development"