ASL is not English. It is an entirely separate visual language with different syntax, lexicon, etc. ASL speakers can communicate to each other over mobile devices using text in the same way that, say, 17th century intellectuals communicated using Latin, or late-dynasty Chinese officials communicated using Classical Chinese.

I do not know how to sign, but I do imagine that a member of the signing community would not feel truly at home in their digital life in that they cannot type the language that they "speak" and very probably think in, but must rather resort to a second auxiliary language whenever they interact with text.

Like, imagine if Apple (or Google or anyone else--this is an industry-wide issue) made it technically impossible for you communicate in anything except French. In this hypothetical world it is not a show stopping issue because you are fully proficient in French having used it in some way nearly every day of your life, and so are all the people you would want to communicate with. But it's not your mother tongue, and so you wouldn't really feel at home or fully included in the digital world, now would you? Texting your family and close friends in French when in fact all your other interactions with them are using spoken English would just be weird.

(Incidentally I do wonder if Swiss German or Scotts speakers feel similarly, and if they don't to what extent that serves as a counter point.)