M A K Halliday would say Keats' clause is a relational process -- precisely, an intensive process of the identifying type. We cannot tell which is the token and which is the value, which is the identifier and which is the identified, whether the clause is active or passive.
So, is it Truth which is Beauty, or Beauty which is Truth? Keats switched them round in the second half of the line. But still, strictly grammatically speaking, both Truth and Beauty could equally be the subject in each case.
None the less, I love that line. It's one of the most beautiful thoughts ever put into words. And it's highly evocative just because it's vague. Giacomo Leopardi (an Italian poet who lived in the first decades of XIX century) wrote that "vague and indefinite expressions are the most poetic ones."