Q: Do the Upaniṣads talk about or mention mithyā? If not, why not, when Advaita seems to speak so much about it?
A: The absolute ‘bottom line’ of Advaita is as expressed by Māṇḍūkya Up. and Gauḍapāda’s kārikā-s, namely that there is no creation, no one has ever been born etc. Māṇḍūkya 7 is the final word on the matter:
“This (consciousness) is known as the ‘fourth’. (It is) neither (the knower of) the internal (world), nor the external. Neither (is it the knower of) both. (And it is) not (just) a ‘mass’ of consciousness. (It is) not consciousness (in the empirical sense of conscious ‘of’) nor (is it) unconsciousness. (It is) imperceptible, transaction-less, not ‘graspable’, un-inferable, unthinkable, and indescribable. (It is) the essential ‘I’-experience. (It is) the negation of the experience of all plurality of the universe. (It is) pure, tranquility, and non-dual. This is the Self. This is to be understood.”
Consequently, anything in experience (i.e. dualistic) cannot be real. Yet we DO experience the world. Therefore, it has to be mithyā. No need to specifically talk about it. Gaudapada does, in fact, in Chapter 2, which is called ‘Vaitathya Prakaraṇa’. Vaitathya is essentially a synonym for mithyā. (My book ‘A-U-M’ is all about this – https://www.advaita.org.uk/extracts/a_u_m_unreal.html).
Ṥaṅkara also talks about it in BSB 1.4.19; 2.1.14; bhāṣya on Mand. Up. 7; Gaud. kārikā 4.9 and Vivekacūḍāmaṇi 194 -5 (ish).
The distinction between paramārtha and vyavahāra is also effectively another way of talking about mithyā. Vyavahāra is ‘appearance’, whose substantive reality is actually Brahman. Every discussion about ‘name and form’ as opposed to reality is about mithyā, whether or not the word is used.