Google has released a new link attribute specifically for use in forums to identify links posted by forum members, it's called rel="ugc". Here's what they say:
rel="ugc": UGC stands for User Generated Content, and the ugc attribute value is recommended for links within user generated content, such as comments and forum posts.
More details @ Google's blog: https://webmasters.googleblog.com/2019/09/evolving-nofollow-new-ways-to-identify.html
This issue is for phpBB devs to consider adding the rel=ugc to the core phpBB code to help Google properly index forum posts following standards.
It might be a good idea to have rel=ugc NOT show up for forum posts linking to the actual site domain since that content is already verified as good. rel=ugc should probably just be used for external domain links since that content is outside the control of the forum owner.
Google also says: "All the link attributes, sponsored, ugc and nofollow, now work today as hints for us to incorporate for ranking purposes. For crawling and indexing purposes, nofollow will become a hint as of March 1, 2020."