What has Siva said about the caste or varna system?
I will post some references where Lord Shiva spoke about Varnasrama Dharma.
In SECTION CXLIII, Anushasana Parva of Mahabharata, Maheshwara says to Goddess Uma that varna comes by birth is just for sake of classification and later conduct alone decides Varna and Varna can be changed.
Neither birth, nor the purificatory rites, nor learning, nor offspring, can be regarded as grounds for conferring upon one the regenerate status. Verily, conduct is the only ground. All Brahmanas in this world are Brahmanas in consequence of conduct. A Sudra, if he is established on good conduct, is regarded as possessed of the status of a Brahmana. The status of Brahma, O auspicious lady, is equal wherever it exists. Even this is my opinion. He, indeed, is a Brahmana in whom the status of Brahma exists,--that condition which is bereft of attributes and which has no stain attached to it. The boon-giving Brahma, while he created all creatures, himself said that the distribution of human beings into the four orders dependent on birth is only for purposes of classification.