You may not have as much to worry about in terms of security as some previous comments imply.
Note that iOS 14 is a bit of an anomaly. This is the first time that I can remember that Apple has continued to support and released iOS updates for two systems, either of which can be run on an iPhone. In other words, if you have iOS 14.5 or newer you can still update iOS 14 and are not obliged to upgrade to iOS 15 in order to get security updates. In the past you would have been required to upgrade to iOS 15. There would be no more updates for iOS 14 since anything that can run 14 can also run 15.
From what I can tell with my device running iOS 14.8, in doing this Apple is essentially releasing updates for security issues with iOS 14, but not feature updates; for those you need 15. Therefore in terms of security you are not as far behind iOS 15 as you would, say, be if you were running an older iOS version that wasn't receiving any kind of updates. The security aspects you might be missing out on would be anything inherent to a feature of iOS 15 itself, not not a reported software issue as such.
Some of this is discussed in this document:
Update your iPhone, iPad, or iPod touch. Click here --> https://support.apple.com/HT204204
Of course the Apple mantra is you should always be running the newest iOS version your equipment will support. iOS 14 still receiving security updates (at least for now) is quite a surprising change in past policy. As for the features, that is a matter of choice if you feel you need those.
