Ken Ekholm Please see below response to your queries.
Do they still manufacture Azure Sphere MT3620 devices or is there any replacement device in the near future?
Microsoft works with a diverse group of expert hardware manufacturers and their broad ecosystem of vendors to design specialized Azure Sphere–certified chips that meet the needs of various markets. Extensive research and testing contribute to the development of the security technology and architecture for each chip.
What do you mean by replacement? It is still available for purchase. See Get started with Azure Sphere and What's new in Azure Sphere
Are there ny other similar devices like Azure Sphere MT3620 that can be used with Azure Sphere?
Please elaborate your scenario and specific needs so we can assist you accordingly. I suggest you, refer IoT device selection list and see if that helps.
If this answers your query, do click Accept Answer and Yes for this answer as helpful. And, if you have any further query do let us know by commenting in the below section, happy to help!