Is It possible to update an old Mac to the latest iOS version?

Possible workarounds for not being able to upgrade...

Mostly you just need a more modern Browser than Safari, like Firefox or Brave.

Thanks to A Brody, Browsers by Mac operating system…

Browsers by Mac operating system - Apple Community

You'll need to give more information about what (and with what) you're trying to achieve. Community member @John Galt wrote a helpful user tip about how to best go about "Writing an effective Apple Support Communities question".

You can see it here:Writing an effective Apple Support Commun… - Apple Community