@Alessandro Pantano Thanks for reaching out. mtv1 is related to Consumption SKU which cannot be vnet integrated and is a shared resource. so completely different.
STV1 uses classic cloud Virtual Machines, while STV2 uses Virtual Machine Scale Sets which is a better environment with added security and resilience.
APIM consumption (mtv1) are not impacted by the stv1 retirement on the 31st of August.
please refer the similar discussion here: https://learn.microsoft.com/en-us/answers/questions/1803939/api-management-mtv1-getting-retirement-notice
do let me know incase of further queries, I would be happy to assist you.