Forum signatures are a topic that have come up before. The last time was a little under five months ago, at the time I ran the topic passed the other moderators and pretty much everyone was against the feature. I then created a rather long, but detailed post outlining why were not for signatures. You can read that post here.
It has been some time since this discussion was had, I don’t foresee it having a different result, but if you want to request the feature create a topic in General Discussion and use the tag #forum-feedback.