If you only had update Jenkins version without plugin it's strange. I would suggest see Jenkins logst know if some plugin failed to start.

Please write the plugin version you are using.

We use latest LTS version in production.

Anyway there is only "Publish xunit test report..." as post-build step action (it is a publisher).
If you are sure did not have update xunit plugin version this is not an issue.
There was a build step in version 1.x than deprecated in version 2.x for over two years with a warning message in the jenkins build log and removed in 3.x (dicember 2020).
If you had upate plugin from 2.x to 3.x and never fix the warning replacing the build step with the post-build step (like junit plugin) than I will reject the issue (in two weeks).

Thank you for your fast response.

The plugin in version I was previously using was 1.102, with Jenkins 2.150.3, so this would explain why I was able to add the build step.

I am now using xUnit 3.0.2. Jenkins 2.289.2

I would be extremely grateful if the  build step could be restored.

Having the "Process xUnit test result report'" build step is very useful, since it allows me to stop my build immediately after the failure of my unit tests.

My build job first builds then runs the unit tests. If the unit tests succeed, I proceed with the rest of the build. If they fail, the build is aborted.

Thank you.

Migrate your jobs to pipeline or install a conditional build step plugin. I will not enable this build step anymore (specially after wait 3years).