MacBook Air 2012 and no longer support newest MacOS?

If you're on Sierra now, as your tag line seems to indicate, you can upgrade to High Sierra, Mojave, or Catalina. If you upgrade to Catalina,

• 32-bit applications will stop working.
• iTunes will go away, replaced by separate apps: What happened to iTunes? - Apple Support
• You might be able to run current versions of applications such as Firefox, LibreOffice, and the Affinity V2 suite.

How to download and install macOS - Apple Support

That model can't be upgraded past Catalina.

(246979)

Just because your mid-2012 Air could run Catalina and no later, does not mean that it has the resources to do so that result in a satisfactory user experience. If your Air has 8GB RAM, it might be satisfactory, but the standard SSD storage according to a Black Magic benchmark caps at 203 MB/s for the 512 GB drive configuration. Catalina will make you painfully aware of that slow drive.

Mostly you just need a more modern Browser than Safari on Catalina, Mojave, or High Sierra like Firefox or Brave.

https://brave.com/

https://www.mozilla.org/en-US/firefox/83.0/system-requirements/

Firefox 83.0 System Requirements (mozilla.org)

Opera…Thanks

https://www.opera.com/download/requirements

Opera system requirements | Opera

Thanks to dialabrain…

Firefox 115 is now the current version. runs in High Sierra.

https://www.mozilla.org/en-US/firefox/new/

Thanks to A Brody, Browsers by Mac operating system…

Browsers by Mac operating system - Apple Community