The only way to upgrade Safari (from that far back) is to upgrade the OS. Apple generally supports the current OS and two back. Meaning, Ventura, Big Sur and Monterey will get new versions of Safari as Apple releases them, but Mojave will only get security updates.
But you don't have to subscribe to Microsoft 365. There's also the one-time fee, perpetual license Microsoft 2021. However, even those versions now have a catch. Think of how long Office 2011 lasted. From November 2010 all the way through High Sierra (and only worked in Mojave as a hit-or-miss thing).
Anyway, that was 7 years of support from MS for Office 2011. Software companies stay in business by selling you new versions, not supported old ones for free in perpetuity. The perpetual release versions of Office are now fixed at three versions of macOS. This is Microsoft's text for Microsoft 2021:
Starting with the November 2022 update (16.67) macOS Big Sur 11 or later is required receive updates to Word, Excel, PowerPoint, Outlook and OneNote. If you continue with an older version of macOS, your Office apps will still work, but you'll no longer receive any updates including security updates.
Is macOS Ventura 13 supported?
Yes. For the best experience with the new operating system, make sure that you have installed the latest updates for Word, Excel, PowerPoint, Outlook and OneNote.
Note the three macOS release span. Big Sur, Monterey and Ventura. Whatever name Apple gives the next OS release after Ventura, MS 2021 will stop working if you install the next new OS. You'd have to stay in Ventura, or upgrade to the next perpetual release of MS Office, if there is another such release. MS has been saying for at least a couple of years that the 365 subscription version will be the only way to purchase Office. But they've, so far, strung out the perpetual versions a bit longer.
