Following thanks to all contributors, the blog notes:

Most applications that worked with OpenSSL 1.1.1 will still work unchanged and will simply need to be recompiled (although you may see numerous compilation warnings about using deprecated APIs). Some applications may need to make changes to compile and work correctly, and many applications will need to be changed to avoid the deprecations warnings.

And points out a couple of new features:

OpenSSL 3.0 introduces a number of new concepts that application developers and users of OpenSSL should be aware of. An overview of the key concepts in libcrypto is available in the libcrypto manual page.

A key feature of OpenSSL 3.0 is the new FIPS module. Our lab is testing the module and pulling together the paperwork for our FIPS 140-2 validation now. We expect that to be submitted later this month. The final certificate is not expected to be issued until next year.

And finally, LWN notes on the license change:

OpenSSL has also been relicensed to Apache 2.0, which should end the era of “special exceptions” needed to use OpenSSL in GPL-licensed applications.