Since I don't know any details, I'll speak generally. Ideally speaking, it's not really a good response to unfriend / block someone just because they happened to be busy dealing with whatever was happening in their lives for a time. Ideally, a friend would have tried to be understnading and reached out to you before going through with such an action. And what I mean by "ideally", it's when both people's views of their friendship are very similar, if not the same. Simply speaking, when they care about you as much as you care about them and when they don't expect more than what each can give the other. In other words, it's like a balanced scale.
But such ideal relationships can be hard to come by. More often than not, a person is bound to experience uneven relationships where one side cares more than the other, expects more than what the other person is willing to give (like to talk to them even though they're too busy to do so) and generally holds the friendship to a different standard than the other, causing the metaphorical scale to be uneven.
Having said that, I don't think such uneven relationships are innapropriate, including what happened to you. Caring for everyone equally is practically impossible and even if you could actually do so, the chances that the other person can too are even lower. In this particular case, it sounds like you cared more about your friend than vice versa, and the friendship presumably gradually lost its meaning to them while you weren't talking, since that's what probably gave the friendship meaning from your friend's perspective. (Ideally, a friendship shouldn't be based on being able to talk all the time) It's certainly sad when such is the case but it's a common and natural occurence. It's just something to accept and move on hoping that your future friendships will be more even / balanced / ideal, with people caring about you as much as you do about them and not expecting things things, which you're not willing or available to give, from you.
In conclusion, I would consider it a bad response in an ideal / even friendship but possibly a natural one in an uneven one, depending on the circumstances.
Sorry for the long comment, I hope that it was at least even a tad helpful in providing you with an answer to your question. Nothing was meant to be personal either towards you or your friend, it's just conjecture along with my general opinion on the subject.
Thank you for the amazing in-depth response, this hit the nail on the head for me. I see everything as if it's on a scale and I don't believe the punishment of unadding me is justified by the means.
PS: this thread is to shade another member of the forum, lol.