So it depends a lot on age, location, and severity of hearing loss. First Deaf and deaf aren’t the same thing. Capital D Deaf means culturally Deaf regardless of severity of hearing loss. Someone can be hard of hearing and Deaf and someone who hears worse may not be. Hoh and deaf people who aren’t Deaf usually are either late deafened or were mainstreamed (basically given hearing aids and usually not taught sign). Little d deaf implies hearing loss severe enough that it would require yelling to have an unaided verbal conversation (not precise but it’s generally accurate).
So for starters, if you grew up with the internet you’re probably fluent in written language, probably English. Unless you’re from a time and place where hearing people weren’t literate you’re probably literate as a deaf person. And to my knowledge there are no widely adopted sign language written forms beyond all caps words in language common to area with sign syntax. That’s why when you see Deaf people struggling with verbal languages it’s usually issues of spelling or syntax not vocabulary. But there’s also fingerspelling. There are a lot more words than signs. Common words quickly get a sign, but shit you don’t have to express face to face often to another Deaf person like professional jargon, you just spell it out.
But think about the 80s-90s in America. People who didn’t learn to read in school were still around. Especially if the system didn’t really care and their parents were illiterate. Even now there are people who can read but not at speaking speed. Captions are fine now, it’s like English to the Dutch. You’ve been using it in entertainment at least partly your whole life. But when it was a language someone half bothered to teach you at ten not so much