I have never seen this before, but because most of the in-flight safety instructions appear to be replayed recordings these days (at least in my experience for both domestic and international airlines), I would like to find out how this information is being provided to deaf or hearing impaired passengers. I have never seen a sign language interpreter onboard a flight, so I wonder if this is something that people on this site know about.
Your question is basically no different at all to any situation involving a passenger that does not understand the language that the demo is conducted in, which is why passenger carrying aircraft above a certain capacity are required by most national aviation regulatory bodies to supplement the demo with a flight safety card with illustrations depicting the same info as the demo.
Every seat has a flight safety card in the pocket in front of it, thats enough to satisfy the legal and regulatory requirements for the airline.
In addition to the safety information card, many flights with in-flight entertainment systems will play a video to provide the safety briefing. It's common for this video to include subtitles, sometimes in multiple languages, for accessibility.
For example, Delta Air Lines:
Passengers can simply watch the video and read the text instead of listening to the audio.
I'd add that it's generally understood and accepted that not every passenger on a flight will be able to understand the safety briefing. I've been on a number of flights where the briefing is only conducted in a language or languages I do not understand, and the airline is not normally violating any regulation by failing to provide me with a translator.
Note that airlines may require passengers travel with a "safety assistant" if they aren't able to receive the safety briefing through some means or another, such as "both severe hearing and severe vision impairments."
Airports and Airlines are under pressure to make their materials and communications accessible to the Deaf, mostly because of the ADA laws. They have improved, but this doesn't mean that they are perfect. Many are working with Deaf organizations such as NAD to improve accessibility and other organizations offer advice or interpreters for the Deaf such as DeafEurope in cases where companies are not so "Deaf Friendly."