According to Microsoft, “As of the Microsoft 365 for Mac November 2020 update, macOS 10.14 Mojave or later is required to receive updates to Word, Excel, PowerPoint, Outlook and OneNote. If you continue with an older version of macOS, your Office apps will continue to work but you will no longer receive any updates including security updates.”
If you can’t upgrade to a newer version of macOS or get a new Mac computer, Microsoft does mention that your apps will continue to work, it’s just that they will no longer be able to receive any updates for new features or security. If you don’t have any problems with using a potentially outdated piece of software, we suppose this isn’t really that big of a deal.
However, if you’re the sort that needs to be updated at all times for work or for school, then this could be an issue. It is a pity that High Sierra support is being dropped considering that it’s not that old (it was released in 2017), but it’s something you’ll have to take into consideration.