When will the IMAC 2019 RETINA 4K 1-5 Be unsupported?

Since it's capable of running macOS 15 Sequoia, it should still be supported until such time as macOS 18 is released, at which point (assuming it does not qualify for macOS 16) it would roll off of the supported OS list.

Apple, and most developers, typically support the current and two previous major releases of macOS.

That would be approximately fall of fall of 2027. Of course, it is possible that it might get macOS 16, which would push it out another year.

Depending on what you want to do with it, supported and useful may be two completely different things.

There's no way to know at this time. We'll see whenever the first beta of macOS 16 is released.

In this case dialabrain is most correct. No one here (we are all end users just like you) so we have not special insight into Apple's plans. As a general rule of thumb it is a good idea to upgrade to a newer Mac every 7-8 years or so.

FWIW, until Apple releases the information, anything else is just speculation.