How frequently do PaaS:VPS agencies upgrade OS_release in comparison to PaaS:shared-webserver?
From my experience this is very provider specific and requires evaluating the SLA offered for a particular service.

For example the siteground cloud server offerings shown at
https://www.siteground.com/cloud-hosting.htm state: We preinstall and maintain all the software you need on your server.

While offerings from liquidweb they make it clear they will customize the support level to fit your requirements (the custom levels of management are detailed here). You can chose the support level for the product you purchase, whether it's a cloud VPS or dedicated server hardware. 

IMHO most cloud products will probably be vendor managed and provide you a way to perform needed customization. But general statements are not useful when it comes to spending money. You have to confirm what your vendor is providing.