Maybe it will be a good option to make a setting which will allow to make profile pages as indexable by search engine, so every community administrator will decide by themselves do they want to allow profiles to be open for search engines or not.
In order to prevent spammers to use such profile pages and discourse forum/community as their link building platform, it might be a good idea to add another option: forum profiles allowed to be indexed (can be restricted via meta tag) for users who meet certain criteria: level of trust (such as in setting related to wiki posts), or being a member of some group of verified experts for example. Some communities might have real experts such as doctors, lawyers etc who will be more motivated to participate in conversations if they can have an indexable profile page. They might be experts in their field but they don’t know anything about SEO, web, etc. Let’s say they don’t want to make some blog, website or something like that. Short link to their profile at some discourse community might be a good option for them.
Although profile pages are not exactly a ranking factor but it helps search engines to understand is this particular article or forum post is trustworthy: