Sometimes what a person wants is not what is best for them. As an employer, you don't know what treatment a person with social anxiety disorder may be getting, but it is entirely possible that their therapist would consider reducing spoken communication as stepping backwards in their treatment.
And, since you are not the therapist, the best thing is for the employee to request specific accommodations with a note from the therapist/doctor. Once the employer gets this request, then they need to decide if they are reasonable, if spoken communication is an essential job function, and the job cannot be done in any other way.
For a software engineer, a lunch-and-learn where they have to present to their teammates is not usually an essential job function. Talking to customers to figure out exactly what is needed might be considered essential, and might not be. That can depend on the level of their written communication, as well as that of the customers. If the customers are unable to write clearly, and requirements gathering is an essential function, then at least listening to the customer may be an essential job function. If someone else can do the requirements gathering, then perhaps it could be a reasonable accommodation. It very much depends of the specific company and people within it.