There is the notion of Krama Mukti - going towards nirguna Brahman in stages - but once the veil of Maya is pierced, does Advaita say the deities are no longer real?
The Saguna Dieties are nothing but Nirguna Brahman under the effect of Maya. That is, as long as we think 'we are the body', the Brahman appears as Saguna and is real from relative standpoint. The moment self-realization happens i.e we realize 'I am Brahman', the same dieties show up in thier absolute form - as Sat-Chit-Anand form.
As Ramana Maharshi says, "Bhakta and Jnani's path are one and the same. What is Atma or Brahman for Jnani is the God of Bhakta".
The Rig Veda I.164.46 says
indraṃ mitraṃ varuṇamaghnimāhuratho divyaḥ sa suparṇo gharutmān | ekaṃ sad viprā bahudhā vadantyaghniṃ yamaṃ mātariśvānamāhuḥ ||
They call him Indra, Mitra, Varuṇa, Agni, and he is heavenly nobly-winged Garutmān. To what is One, sages give many a title they call it Agni, Yama, Mātariśvan.
The deification of the same Almighty into different forms, perhaps, necessitated when the humans could not grasp the Almighty as the pure formless.
The deities are instrumental in driving an inquisitive mind towards the ULTIMATE SOURCE of Power behind the Universe.
The deities are REAL, as long as the power behind the deities is not understood. The deities represent the Almighty in Human/Animal/Human-animal/inanimate form, ie., Shiva, Adisesha, Nrisimha, Agni, etc.
Once the source of power behind the deities is understood, one need not go through "n" number of scriptures, argue/discuss, because the source is realised.
The following is an excerpt, which is relevant to the question, from the teachings of Sri Ramana Maharshi.
A certain wealthy man’s house was closely guarded. It had also a ferocious dog chained to a pillar at the gate. The dog and the chain were however very skilful pieces of art.
They were sculptured in stone but appeared life-like.
A pedestrian on the road once took fright at the sight of the ferocious animal and hurt himself in his attempt to dodge it.
A kindly neighbour took pity on him and showed him that it was not a living dog. When the man passed by it the next time he admired the skill of the sculptor and forgot his old experience.
Thus when he found it to be a dog, he could not see the stone of which it was made; and again when he found it a piece of sculpture he did not see any dog to hurt him.
Sankara writes in his commentary on Brahma Sutra Bhasya that Brahman appears to be many due to limiting adjuncts. That would suggest that in his view Saguna Brahman is not ultimately real.
Since this Self is by nature Consciousness Itself, distinctionless, beyond speech and mind, and can be taught by way of negating other things, hence in the scriptures dealing with liberation an illustration is cited by saying that it is "like the sun reflected in water". Here the aspect kept in view is the one with attributes, which is not real and which is created by limiting adjuncts, as it is done in such texts, "As this luminous sun, though one in itself, becomes multifarious owing to its entry into water divided by different pots, similarly this Deity, the birthless, self-effulgent Self, though one, seems to be diversified owing to its entry into the different bodies, constituting its limiting adjuncts." Similarly, 'Being but one, the Universal Soul is present in all beings, though One, It is seen as many, like the moon in water" (Amritabindu, 12) and other texts.
Brahma Sutra Bhasya of Sri Sankaracharya, III.II.18
Sankara also says that Saguna Brahman is for purpose of meditation.
Brahman is only formless to be sure, for that is that dominant note (of the Upanishadic teaching).
...Hence in sentences of this kind, the formless Brahman has to be accepted. But the other texts, speaking of Brahman with form, have the injunctions about meditations as their main objectives. So long as they do not lead to some contradiction, their apparent meanings should be accepted. But when they involve a contradiction, the principle to be followed for deciding one or the other is that, those that have the formless Brahman as their main purport are more authoritative than the others which have not that as their main purport. It is according to this that one is driven to the conclusion that Brahman is formless and not its opposite, though texts having both the purports are in evidence.
Brahma Sutra Bhasya of Sri Sankaracharya, III.ii.14