Hello Jean-Francois Roger ADM,
Welcome to the Microsoft Q&A forum.
Please follow the below blog post by Brad Harris
https://key2consulting.com/how-to-test-azure-data-factory-linked-services-using-powershell/
Here, instead of Get-AzDataFactoryV2LinkedService, you can use
Get-AzSynapseLinkedService in the script to get the information about synapse linked services.
I hope this answers your question.