The exact date of the reported change is unknown, but a developer at TapeACall, who informed AppleInsider of the modified rules, said the use of special characters was available when they updated their app a little over two weeks ago.
As seen in the image above, Apple is no longer allowing developers to submit app description edits with the unique character sets, which in this case includes a checkmark, explosion, "no symbol" and a speech bubble. It is thought that others are included in the new restrictions, but that has yet to be verified.
Developers sometimes use non-standard characters like checkmarks in bullet lists to draw attention to tweaks or the addition of new features to an app. For example, YouTube's most recent update on April 15 utilized checkmarks to denote the app's basic feature set.
If the new rules are indeed rolling out, Apple has not specified why it is restricting emoji use in descriptions. It can be speculated, however, that the move is meant to create a more unified App Store experience.
Update: Another developer has reported the date of Apple's new iconography requirements to have been set on April 19.