iOS 15.4 is the next major software upgrade for iPhones. It is already available to preview in the form of developer betas. Apple has released four of these thus far, the latest of which add the latest updates to the AirTag system intended to avoid their use for malicious purposes. One new beta also reportedly addresses a user-privacy lapse left in an earlier stable version.
https://www.notebookcheck.net/iOS-15-4-is-slated-to-bring-the-most-keenly-awaited-Apple-features-to-iPhones.604802.0.html