I can understand that signatures are a great place to get creative. But some of the signatures are just annoying and also hurt the site itself. For example we have an active community member that has an image in the signature that is crazy large (dimensions: 3,510px × 1,272px; file-size: 320.5 KB) Even though it is scaled down by the css-rule in the image-tag, it takes up most of the space inside the viewport. If all users would have such an image in their signature the site becomes heavy for people with slow internet-speeds or browsing with an mobile-device.
Like in all growing communities, there is some point where something like public available rules or guidelines are needed. Most of the forums from the time vanilla was "fresh" had such rules, like signatures are not allowed to be larger then 3-5 rows of text (at default size) or max 100px in height.