Is there a SE-site where questions about sign languages are considered on-topic? Or has there ever been a denied/failed Area 51 proposal?
It might depend on what type of question about sign language you have, but in some form it would seem to be on topic on Language Learning. From the Help Center list of topics:
Questions about learning or teaching techniques regarding any human language, including dead, unspoken and constructed languages.
As sign languages are human languages, they should fall under that category. Moreover, there is already a sign language tag with seven questions.
The above should probably be suitable for questions mostly relating to learning/teaching a sign language. Other types of questions might be on topic on Linguistics. The Help Center list of topics there explicitly includes sign languages:
Sign Languages and writing systems
The sign languages tag there has 47 questions.
Questions about sign language as they relate to a particular topic would probably also be on topic on a site that deals with that particular topic. For example if you had a question about using sign language at work, I would imagine that it might be on topic on The Workplace.