Forum signature backlinks can have some impact on keyword ranking, but they are generally considered low-quality backlinks and may not have as much impact as other types of backlinks. Additionally, forum signature links often include a "no-follow" tag, which means that they do not pass on link equity to your site. This means that the link will not help you improve your search engine rankings.
It's important to focus on acquiring high-quality backlinks from authoritative and relevant websites, rather than relying solely on low-quality links from forums or other sources. Building relationships with other websites in your industry and creating valuable, shareable content can help you earn high-quality backlinks that will have a more significant impact on your search engine rankings.