What is the ruling of Islamic law regarding a former employee dealing with clients of his old company, in the absence of a prior agreement preventing that, and his informing the clients of his changed employment status?

There is no objection to your dealing with clients of your former company or others, especially if you inform the clients that you are working for yourself to avoid deception, as the company does not have the right to prevent you from dealing with its customers. Attention must be paid to the Sharia precautions in the field of tourism.