Can i still use the macbook 12 inch from 2015 even though it was discontinued

Yes. That's a great machine as far as screen quality and portability goes. It weighs just a hair over 2 pounds. Even 13" M3 MacBook Airs – the lightest Mac notebooks in the current lineup – weigh 2.7 pounds.

You can't upgrade it to anything higher than Big Sur. Big Sur is not one of the "most recent three" supported by the likes of Microsoft and Adobe. Big Sur is recent enough to run current versions of Firefox, LibreOffice, the Affinity v2 programs (Photo, Designer, Publisher), and other programs that may let you get some use out of the old machine.

Be careful to avoid getting crumbs on the keyboard. Because this Mac is so thin, Apple introduced a new keyboard design – the now-infamous "butterfly key-switch" design. Dirt can get trapped in the "butterfly" key mechanism in a way that is very hard to fix, and replacing the whole keyboard is/was a very expensive proposition due to the other things that must be replaced along with it. Apple once offered a service program for Macs that had malfunctioning butterfly keyboards – but that program has now expired, and Apple shows no signs of renewing it.

This Mac also introduced the Force Touch trackpad – a trackpad that does not mechanically "click", but uses haptic feedback to give you the illusion that it does. Because the Force Touch trackpads did not turn out to have the same problems as the "butterfly" keyboards, Apple continues to use Force Touch trackpads to this day.

Most likely but there some things you should consider. First, be sure that the Mac has been properly prepared following the Apple instructions on the following link.

What to do before you sell, give away, or trade in your Mac - Apple Support

If that process has not been followed, you will then need to have the Mac's password or, very simply, it will be unusable.

Nearly all the Macs I use have been discontinued.

I suspect there is something more to your question. Can you elaborate?