When will it not be possible to upgrade my Mac OS to the latest OS?

The iMac Pro did not exist in 2011.

I'm assuming that what you actually have is a Mid 2011 iMac.

That machine Is now considered Vintage. It can run macOS High Sierra, 10.13.6.

It stopped getting major version updates several years ago.

That's rather difficult to say since you don't tell us what model Mac you have.

When the tech required to do what the new OS needs to do is no longer compatible or viable on your system. There is no set rule.