As a matter of fact, yes. If there has been a preceding call of ungetc(), and that character has not been read, the next call of ungetc() leaves non-binary streams in an undefined state.
In any event, can you name a circumstance in which that particular construct would achieve anything useful .... in comparison, say, with "c = '\n';"
n1570 is a draft. It is not the standard. As a matter of policy, standards committees do not remove features from standards that were in previous versions of a standard and have been in common use, without a process of deprecation (deprecate in one or more version of the standard, before removing it).