I think "obviate" is a better term than "amputate". I mean, who the hell cares about penmanship? Absolutely useless skill.
Similarly you could say remembering phone numbers is a useless skill that many had pre smartphones - you just knew your 30 or so most used numbers. Now we all depend on our phones and once the battery is dead many people don't seem to know any number by heart.
There is Twitter for that
If I tweet, I can shoot out all of that nonsense to the world, secure in the knowledge that the filtering labor will be done in combination by some subset of potential recipients (whose choices to engage or not will provide the fodder that the Twitter feed-building algorithm can use to determine whether it's passed the filter).
This might then indicate that a skill that might be atrophying is the ability to filter one's own writing for relevance/value, not just to produce it in cursive.
I'll leave it as an exercise for the reader to determine what other signs they would look for to confirm or disconfirm this hypothetical atrophy.
Email's interesting because there's traditionally been very little of that social-media-feed-style automatic sorting / filtering outside of spam (not that the "priority inbox" concept isn't trying) so the weight still properly lies on the sender to make the missive easy to handle. I too edit the bejesus out of all my work emails, but it's a different skill than having to compose a coherent first draft without easy edits.
The actual hardest thing to go paperless is math. When they teach strategies and ask to "show your work" there's no easy way to type it.
Reminds me of Tips for Mathematical Handwriting: