No, it is NOT true.
And speaking as a forum moderator who has to deal with the deluge of spam we face every day, this is one bit of outdated advice that I really wish would finally die away.
Any link you can place yourself is (by Google's definition) not
a "high quality" link. Whether you drop a link in a forum post, or you include it in your forum signature, Google is not going to value it highly -- assuming the forum in which you post even allows links to pass value in the first place (many do not).
There is one exception to this: if you are a valuable, active regular
of a forum, it's possible that some of the other forum members might visit your website and find something of value there. If they choose to post a link to one of your pages on their own
website or blog because they like something you posted and want to share it with their visitors, that's called an "earned editorial link." Such links can potentially be valuable.
But in general, plastering your link all over forums is not a viable "link building" tactic.