When will macOS High Sierra certificates expire?

End of January, yes.

I’d expect it feasible to find, manually load, and trust the Messages certificates using Keychain Access app.

Here’s how to fix the Sectigo certificate: Updating the Certificate Trust Store on m… - Apple Community

Here are the certificate trust stores: Certificate Trust Store on iPhone, iPad, … - Apple Community

Fund the Messages cert in a recent trust store, extract it, and add it to the local macOS 10.13 certificate store.

The last local macOS server 10.13 box was retired a few months back, so nothing to test with.

macbookguy12 wrote:

Thank you for the answer. What are sectigo certificates and trust stores certificates?

Given you’re asking a certificate-related question, the links make an attempt to describe how these things work on macOS, and how public certificates can be located and added.

I’ve not specifically gone looking for the Messages public certificate, but I’d expect to find it in a recent Apple trust store source file.

I'm not sure of the exact date, but I believe that it's around the end of next January (January 2027).