I also don't think Google indexes links in signature from this forum so it doesn't matter really. I've never made a search outside the forum for a brand and was redirected to a post on this forum containing the link in the signature of a user. You can do more research about that. Best of luck.
Signature space has its advantage against Avatar, it is links but both Avatar and Signature can help to advertise a brand. Difference is only link that is allowed in Signature space, clickable but in avatar, it is only a text or in personal text and isn't clickable.
I have posted in some threads that got many backlinks and a very high page rank. When I wasn't in a signature campaign, I put my website in my signature and in some days, when I checked my website via ahrefs and semrush, they both showed significant increases in domain authority and I experienced better ranking in Google.
You must write enough posts in that period to get traffics to your website through your signature advertisement around the forum. That is the same with signature campaigns where companies and campaign managers ask people to make a minimum post quota each week to be paid.