After quite some time I accidentally checked the "Pipeline Syntax > Global Variable Reference" for my shared Jenkins pipeline library and saw that the "<pre>" tags are now (unfortunately I can't remember when it was still OK, I just know that it used to be OK for a long time) stripped. Since it is about documentation of usage of shared pipeline library I claim that multi-line coding examples in such "<pre>"tags is fairly common...

Example: For a file called "acme.txt" with the following content

...
    <p>
    Example:
      <pre>
  acme.build {
    node {
      ...
    }
  }
      </pre>
    </p>
...

..., the resulting HTML will looks like:

...
<p>
    Example:
      </p>
  acme.build {
    node {
      ...
    }
  }
...

And using an additional "<code>" inside the "<pre>" section, does not help either. But at least is kept...