My MacBook Air is early 2015 with Yosemite 10.10.5. I've never updated it. Security and blocked access to internet sites is now a serious problem. What's the most recent operating system that is compatible for an upgrade. Other than internet, the apps I use most often are Pages and Photos. Will an upgrade impact significantly on these.
Should your vintage version Safari have security issues that detract
from its ability to upgrade to Big Sur (or to a more modest Mojave
where you may still use the older printer's 32-bit driver software etc)
..There may be a way around those distracting steps to gain newer
macOS. Even a failed Safari may (or not) get past the Yosemite issue.
• How to get old versions of macOS - Apple Support
https://support.apple.com/en-us/HT211683
You may need to upgrade past Yosemite, to slightly less old El Capitan
or High Sierra; before you can hope for upgrade; Catalina, Big Sur, etc.
• How to upgrade to macOS Big Sur - Apple Support
https://support.apple.com/en-us/HT201475
{Those pesky certificates and security issues from late Sept may be at play.}
Should your vintage version Safari have security issues that detract
from its ability to upgrade to Big Sur (or to a more modest Mojave
where you may still use the older printer's 32-bit driver software etc)
..There may be a way around those distracting steps to gain newer
macOS. Even a failed Safari may (or not) get past the Yosemite issue.
• How to get old versions of macOS - Apple Support
https://support.apple.com/en-us/HT211683
You may need to upgrade past Yosemite, to slightly less old El Capitan
or High Sierra; before you can hope for upgrade; Catalina, Big Sur, etc.
• How to upgrade to macOS Big Sur - Apple Support
https://support.apple.com/en-us/HT201475
{Those pesky certificates and security issues from late Sept may be at play.}
..mishchy..
Another option: see if Firefox ESR browser from Mozilla may work in Yosemite.
There's a fairly extensive Firefox page that covers most of their Mac versions.
• How to download and install Firefox on Mac | Firefox Help
https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac
[My old macOS is El Capitan; for awhile, their current 'Firefox ESR' was 78.15.]
And Brave browser has support for older & newer versions. Seems good.
• "Secure, Fast & Private Web Browser w/ Adblocker" - Brave Browser
These both could be of use. Not certain how a newer certificate for your
older browser may fair in Yosemite/Safari. Mine were too old to bother.
Take care & good luck!🌞
My MBP is a 2016 and I also have a 2015 iMac. They both run Big Sur beautifully.
I've had no problems with Apps. Both of them are running Office 2016 Home & Student flawlessly. As does Firefox, and Edge.
The upgrade to Big Sur brought with it the native Apple apps, all of which have zero problems.
MacBook Air security and blocked access to internet sites is now a serious problem
