Learning a sign language is NOT cultural appropriation.
That might be the most backwards possible way of looking at it, and I seriously doubt it has ever been said by an actual Deaf person.
Sign language is necessary for interacting with the Deaf community in any really efficient way, which is the same way it works for any other linguistic minority. Deaf people benefit from as many people knowing sign language as possible.
The only way sign language could be appropriative is if you are only learning it to make fun of it, for example if you only learn how to sign about sex, bathroom use, and profanity. Turning it into an accessory is one definition of appropriation.
Actual appropriation of Deaf culture would look like:
- Giving out sign names in ways that are not culturally expected
- Wearing hearing aids or the exterior parts of cochlear implants as fashion accessories
- Voicing with a fake deaf accent