RE: What does "Modify your extension so it’s compatible with the version 21.1.48089.0" mean?
Hi, This sentence means that when you upgrade to the next major version, it may fail.
Because the extension of your environment may go wrong when installed in the next version.

If you have the full source code of the extension, you can fix it too, as long as you are a slightly experienced developer for BC.
You can go to Extension Deployment Status page to check if your partner has uploaded the next version of the extension.

Schedule should be Next Major.

More details: https://yzhums.com/3591/

Hope this helps.
Thanks.
ZHU