Just to add to the good advise so far...
Possible workarounds for not being able to upgrade...
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/
Details about recent download Brave, for 10.13.6/10.14.6 are here:
<https://github.com/brave/brave-browser/releases/tag/v1.57.64>
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.
119.0.1 is now the current version and requires 10.15 or later.
https://www.mozilla.org/en-US/firefox/new/
Thanks to A Brody, Browsers by Mac operating system…
Browsers by Mac operating system - Apple Community
macOS High Sierra - Technical Specifications
General Requirements
- OS X 10.8 or later
- 2GB of memory
- 14.3GB of available storage to perform upgrade*
- Some features require an Apple ID; terms apply.
- Some features require a compatible Internet service provider; fees may apply.
Mac Hardware Requirements
For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:
- MacBook (Late 2009 or newer)
- MacBook Pro (Mid 2010 or newer)
- MacBook Air (Late 2010 or newer)
- Mac mini (Mid 2010 or newer)
- iMac (Late 2009 or newer)
- Mac Pro (Mid 2010 or newer)
macOS High Sierra - Technical Specifications
https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12