Downgrade Mac OS on last iMac 5K 27"

Because Mac's require a specific build versions and the older version does not support the new Mac.

see > Use the Mac operating system that came with your Mac, or a compatible newer version - Apple Support

You might be able to install the previous macOS version onto an external drive and get it to work. But because of macOS's (EFI, Recovery and Container) complexity, I would not recommend forcing it onto or even into a partition on the internal drive.

Can you tell me why before it was possible to do that and what is preventing it today?

Can you put a date to "before?" I've used Macs since before 1990 and don't recall that being possible with the majority of models.

Also we are all end-users like you and have no insight into Apple's notoriously secret decision process and policies, so "why" can't be answered here without speculation, which the forum Terms of Use forbid.

Macs cannot be downgraded to an older system than the one they shipped with.

vghilardi wrote:

Thank you for your response.

Can you tell me why before it was possible to do that and what is preventing it today?

I don't remember it ever being possible.

Why do you need to run Sierra?

You might be able to find a refurbished Mac with Sierra installed.