Is macbook air 2015 obsolete or vintage???

Here you go: Obtaining service for your Apple product after an expired warranty - Apple Support

Scroll down in the link and click on Mac.

Well that actually depends on what you’re going to use it for. If its just for typical university research, it would be enough. But keep in mind that with todays amount of performance many apps need, Old Macs might not be as fast and smooth as newer models.

Also, older models will most likely stop receiving MacOS updates in the future.

Hope that helps.

Best.

Your machine may work for years, however it will be vintage very soon and will be defined as obsolete within two or three years. You have a cushion, your machine will run the newest MacOS Monterey and Apple will likely continue to support this newest MacOS security-wise for another three years. So you have three more years of official software support for your device.