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This is not a C issue, but really about dynamically allocated memory in general.

<i>Think</i> about it.  When you allocate a chunk of dynamic memory, the heap manager finds an available memory region of the right size (or tells you it can't give you the requested memory), marks it as in-use, and returns you a pointer to it.  When you release the chunk, the heap manager marks it as unused.

What happens to the memory after that is not your business.  You have no guarantee that the memory hasn't been allocated by another thread, implicitly by the run-time library, or whether the address is even still valid in your address space.  It could cause a memory fault if you try to read from it.

<blockquote>Is reading the address returned by a heap allocation function after its deallocation truly an instance of undefined behavior?</blockquote>

Yes!  That's the point of deallocating memory.  You are saying that you're done with it.  That means something else is free to use it, and may have by the time you try dereferencing the pointer again.  Depending on the OS, it may have been marked as invalid memory for you and trigger some kind of trap when you try to access it.

Again, <i>think</i> about it.  How else do you expect it to work?  If you still wanted to access the memory, then you wouldn't deallocate it yet.  Once you do deallocate it, it's not yours anymore.