If you're considering on-premise Exchange Server 2019, the "minimum" recommended RAM memory just for that product is 128GB (!): https://docs.microsoft.com/en-us/exchange/plan-and-deploy/system-requirements?view=exchserver-2019

Exchange Server 2016 minimum RAM is 8GB with 16GB recommended.

This is discussed at https://community.spiceworks.com/topic/2229364-exchange-2019-memory-requirements . Although some as-noted "Exchange experts" are running it with much less RAM, there's speculation that if you have problems and need MS support, and you don't meet the minimum requirements, you're SOL. Or that advanced performance features won't work if you don't have that much RAM.

Let the conspiracy theories begin ...

UPDATE: checking out hardware specs for SharePoint Server 2019, it has this paragraph at the top of the page:

"Important: If you contact Microsoft Customer Support Services about a production system that does not meet the minimum hardware specifications described in this document, support will be limited until the system is upgraded to the minimum requirements."

https://docs.microsoft.com/en-us/sharepoint/install/hardware-and-software-requirements-2019