Not the first in this vein, but beside from another direct contempoary example this is the only other thread to address it as a suggestion solely.
Gestures, Non-verbal communication and related skills
Speaking and SigningIts known with certain creatures such as plump helmet men and trolls that they can't verbalise by their lack of tags, but the lack of speech could become a disability that a dwarf might encounter over its lifetime if affected by the right kind of circumstances, like a non-fatal ripping of the throat or mouth or obscurely specific syndrome effects that leaves them very socially isolated and outside of society onto the slow decline into madness as a result. To skirt around this my suggestion also takes into consideration a few other problems like the way to hire-on plumphelmet people who can't negotiate terms to apply for citizenship or long-term contracts.
[CAN_SIGN] is a proposed secondary "[CAN_SPEAK]" that creates a explicit link for creatures to express to each other non-verbally through (ironically enough) the speaker skill as a measure of how clearly they can communicate their messages. Like crutch walking, the speaker skill comes into effect to grind experience when dwarves lose their initial ability to speak as second preferable.
- The ability to kill and slaughter creatures who can purely sign but lack any sort of able sentience might be linked to a token such as [ETHIC:KILL_INFERIOR_CREATURES] as this range might include primates, who use the token to make themselves useful and haul resources while still remaining a butcherable pet & conversation companion after auto-learning to activate this skill through the training process into becoming a stray.
- Hostile signing creatures don't "speak" at all unless they are co-erced into doing so by interrogation or are pacified into letting their guard down, where relevant, kobolds could be used as practice by a novice sheriff for trying to determine where the rest of their hoard might be as well as to scope their intrigue skill.
- Creatures like trolls can be stripped of their intelligence, and still able to be inducted into the forces of evil with a broader sweep for evil creatures that can sign as war-beasts.
Adventurer handicapsAt the moment every kind of adventurer works in perfect order out of the box, but to trade for a optional disability would be a nice roguelike way to create more challenging setups, within the limitations that choosing a intentional disability would allow you more points allocated to your current adventurer pre-setup & affect how you interact with the world which may lead you to relying more on other members of your party however you choose to recruit them or have them start out with you.
- If the creature is deaf, the rules are reversed and the listener instead responds with sign language to the deaf speaker when within a close distance by default.
- If the creature is blind, they can only react to verbal communication but recieve it 1/3rd more effectively.
Certain obvious activities if you take these tradeoffs will be inaccessible for your character personally, who may be recieving the lions portion of the stats.
Blind and non-extra vision creatures lose all ability to sign if they are simultaneously deaf to being unable to perceive anybody talking to them through normal means. Another benefit to choosing to sign, is that it only affects a unit's vision cone whether they have spotted you for understanding and barely reveals your visibility unlike speaking.