Unfortunately, if you look at wider area of forums as a whole, a very large fraction of them are unmaintained, unpatched, or otherwise not kept safe from drive-by spamming. There are entire forum spamming software packages that will register spam users for dozens of different forums in order to drop spammy links. You can even lease the spamming software by the month. :(
I figure already that you'll eventually consider that a valid signal, but I'm sure you could already check my site and that there must already be some disavowed links, yet you personally admit this is a well-maintained forum.
There is a fear that we are being caught in the crossfire, and it's hard not to believe that quite a few of us are going to get hit by that crossfire.
I've added rel="nofollow" as good-enough when trying to help out people throwing too-far of a net.
It isn't, and never has been.
We operate a nursery period ( https://www.lfgss.com/thread9350.html ) in which the actions of new members are both severely restricted and observed by existing members, and all initial content must be 100% public and observable by members (no private messages, no user profile customisations).
We very aggressively remove content and ban anyone who creates any content that appears spammy and continue to encourage reports of spam even long after they have been registered.
We report those individuals to http://www.stopforumspam.com/ at the same time preventing signups from anyone that SFS reports as coming from an IP recently used to spam.
Comments are run through the Akismet API and moderated if they are flagged as spam.
When a spammer is caught (a few times a year someone does get past all of this - for a very brief moment), then every piece of content of theirs, and any other content from nearby IPs is reviewed and usually removed.
We made profile pages non-public, we even removed the customised profile fields from display (though we do leave them in the form as it helps us spot any scripts that are trying to spam us and we autoban them).
To put it mildly, we are aggressive.
We've made a really poor experience for new members of our community to help ensure the quality of the content on the site and that we're free from spam. We felt that was worth it from day one, we still feel that it's worth it.
Which is why it's hard to feel good about Google when our reward for fighting spam on our corner of the internet is to be spammed by merchants whipped into a state of panic by Google.