CRM Portals require high availability. Irrespective of whether you purchase an additional Portal to use in a sandbox context, when Microsoft apply an upgrade it is universally applied to all Production and Non-Production Portals that you have.
This does NOT give anyone any time to test/validate changes that Microsoft implement, which may adversely affect the operation of the Portal; nor does it allow you to test your own implemented Portal customisations.
This can (and has) caused some significant customer issues, where the Portal has been unavailable/not operational for a period of time, until a fix can be applied (from Microsoft or self). Depending upon the Portal use case this can have significant financial and business impacts on an organisation when the site is non operational.
Please can consideration be given to provide a mechanism where a CDU experience (or similar) to provide a period of time to test and validate Portal changes before applying them to the Production environments.
Thanks you for the suggestion, we discussed this within the team and have rolled out a process of early testing every release in your non production (dev/test) environments before it goes GA and gets applied to your production environments.