In the strucured data of each Post there is an url to the authors page:
<div id="post_123" itemscope="" itemtype="http://schema.org/DiscussionForumPosting" class="topic-body crawler-post">
<div class="crawler-post-meta">
[…]
<span class="creator" itemprop="author" itemscope="" itemtype="http://schema.org/Person">
<a itemprop="url" href="https://meta.discourse.org/u/codinghorror">
<span itemprop="name">codinghorror</span>
</a>
(Jeff Atwood)
</span>
These author pages are blocked by http-header noindex. Google wants all structured data to be accessible - see General Structured Data Guidelines | Google Search Central | Google Developers
The authors url is recommended by Google – see Learn About Article Schema Markup | Google Search Central | Documentation | Google Developers
Should Discourse hide the authors url?
Hiding the url might be done by using the property itemid to link all posts from the same author to the same author object without setting any url.
Current state:
<span class="creator"
itemprop="author" itemscope="" itemtype="http://schema.org/Person">
<a itemprop="url" href="https://meta.discourse.org/u/codinghorror">
<span itemprop="name">codinghorror</span>
</a>
(Jeff Atwood)
</span>
Remove property url and add identifier via property itemid - additionally set rel="nofollow":
<span class="creator"
itemid="https://meta.discourse.org/u/codinghorror"
itemprop="author" itemscope="" itemtype="http://schema.org/Person">
<a rel="nofollow" href="https://meta.discourse.org/u/codinghorror">
<span itemprop="name">codinghorror</span>
</a>
(Jeff Atwood)
</span>
