Is the following C union access pattern undefined behavior?

From 6.2.6.1 §7 :

When a value is stored in a member of an object of union type, the bytes of the object representation that do not correspond to that member but do correspond to other members take unspecified values.

So, the value of plugh.i would be unspecified after setting plugh.c.

From a footnote to 6.5.2.3 §3 :

If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ‘‘type punning’’). This might be a trap representation.

This says that type punning is specifically allowed (as you asserted in your question). But it might result in a trap representation, in which case reading the value has undefined behavior according to 6.2.6.1 §5 :

Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined. 50) Such a representation is called a trap representation.

If it's not a trap representation, there seems to be nothing in the standard that would make this undefined behavior, because from 4 §3, we get :

A program that is correct in all other aspects, operating on correct data, containing unspecified behavior shall be a correct program and act in accordance with 5.1.2.3.

Defect report 283: Accessing a non-current union member ("type punning") covers this and tells us there is undefined behavior if there is trap representation.

The defect report asked:

In the paragraph corresponding to 6.5.2.3#5, C89 contained this sentence:

With one exception, if a member of a union object is accessed after a value has been stored in a different member of the object, the behavior is implementation-defined.

Associated with that sentence was this footnote:

The "byte orders" for scalar types are invisible to isolated programs that do not indulge in type punning (for example, by assigning to one member of a union and inspecting the storage by accessing another member that is an appropriately sixed array of character type), but must be accounted for when conforming to externally imposed storage layouts.

The only corresponding verbiage in C99 is 6.2.6.1#7:

When a value is stored in a member of an object of union type, the bytes of the object representation that do not correspond to that member but do correspond to other members take unspecified values, but the value of the union object shall not thereby become a trap representation.

It is not perfectly clear that the C99 words have the same implications as the C89 words.

The defect report added the following footnote:

Attach a new footnote 78a to the words "named member" in 6.5.2.3#3:

78a If the member used to access the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called "type punning"). This might be a trap representation.

C11 6.2.6.1 General tells us:

Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined.⁵⁰⁾Such a representation is called a trap representation.

Other answers address the main question of whether reading plugh.i produces undefined behavior when plugh was not initialized and only plugh.c was ever assigned. In short: no, unless the bytes of plugh.i constitute a trap representation at the time of the read.

But I want to speak directly to a preliminary assertion in the question:

Most C compilers nowadays will have sizeof(char) < sizeof(int), with sizeof(int) being either 2 or 4. That means that in these cases at most 50% or 25% of plugh.i will have been written to

The question seems to be supposing that assigning a value to plugh.c will leave undisturbed those bytes of plugh that do not correspond to c, but in no way does the standard support that proposition. In fact, it expressly denies any such guarantee, for as others have noted:

When a value is stored in a member of an object of union type, the bytes of the object representation that do not correspond to that member but do correspond to other members take unspecified values.

(C2011, 6.2.6.1/7; emphasis added)

Although this does not guarantee that the unspecified values taken by those bytes are different from their values prior to the assignment, it expressly provides that they might be. And it is entirely plausible that in some implementations they often will be. For example, on a platform that supports only word-sized writes to memory or where such writes are more efficient than byte-sized ones, it is likely that assignments to plugh.c are implemented with word-sized writes, without first loading the other bytes of plugh.i so as to preserve their values.