A forum signature can be perfectly normal.
A person writes twenty useful replies about vintage radios and signs each one with a link to their repair blog. Nobody needs to call the police.
The problem begins when the conversation becomes camouflage for the link.
Google’s current spam policies give an unusually specific example under link spam: forum comments containing optimized links in the post or signature. The policy also treats paid links and automated link-creation services as manipulative when their purpose is to influence ranking.
That describes an old but important internet labor market.
Participation can be manufactured around the placement
A promotional link inside a forum signature has one advantage over a naked advertisement: it looks attached to a person participating in a community.
That appearance can be earned honestly. Longtime members often link to a personal site, business, project, or profile in their signature because it identifies them.
It can also be manufactured.
A worker or automated system can create accounts, make short replies, and leave the same commercial link beneath each post. The text above the signature may exist mainly to keep the account alive long enough for the placement to remain visible.
At scale, the operation stops resembling conversation and starts resembling distributed ad inventory assembled one forum account at a time.
The host supplies the domain reputation, the audience, the moderators, the archive, and the surrounding legitimate discussion.
The promoter supplies the link.
A link is not spam because it is commercial
The distinction matters.
A mechanic answering a repair question and linking to a detailed guide they wrote may be genuinely useful. A software developer with a project URL in a signature may simply be identifying themselves. A sponsor can buy advertising transparently.
The stronger signs of abuse are repetition, irrelevance, deceptive participation, optimized anchor text, large numbers of accounts, or posting behavior that makes little sense except as a way to place links.
Google recommends that sites mark untrusted forum and comment links with rel="ugc" or nofollow, and its guidance on user-generated spam encourages moderation, reputation requirements, and restrictions on links from new accounts.
Those defenses attack the incentive rather than the person.
If a signature link no longer buys much search value, there is less reason to employ an army of fake conversationalists to plant it.
Spam Empires are often imagined as giant mail servers blasting millions of messages.
Sometimes the factory floor is much quieter.
It is a thousand people typing “Great point, thanks for sharing” so the real payload can sit underneath their name.