Nibbana is unconditioned (asankhata).
Now at that time the Buddha was educating, encouraging, firing up, and
inspiring the mendicants with a Dhamma talk about extinguishment.
Tena kho pana samayena bhagavā bhikkhū nibbānapaṭisaṁyuttāya
dhammiyā kathāya sandasseti samādapeti samuttejeti sampahaṁseti.
But since there is (atthi) an unborn, unproduced, unmade, and
unconditioned, an escape is found from the born, produced, made, and conditioned.
Yasmā ca kho, bhikkhave, atthi ajātaṁ abhūtaṁ akataṁ asaṅkhataṁ,
tasmā jātassa bhūtassa katassa saṅkhatassa nissaraṇaṁ paññāyatī”ti.
Ud 8.3
There are many quotes to express this, such as:
Birth... Old age and death are impermanent, conditioned, dependently originated, liable to end, vanish, fade away, and cease.
Jāti... Jarāmaraṇaṁ, bhikkhave, aniccaṁ saṅkhataṁ paṭiccasamuppannaṁ khayadhammaṁ vayadhammaṁ virāgadhammaṁ nirodhadhammaṁ.
SN 12.20
the unborn, unaging, unailing, undying, sorrowless, uncorrupted supreme sanctuary from the yoke, extinguishment
ajātaṁ... ajaraṁ... abyādhiṁ ... amataṁ .... asokaṁ ... anuttaraṁ yogakkhemaṁ... nibbānaṁ
MN 26
Also, while Nibbana is unconditioned, it exists. The suttas say:
There exists, mendicants, that sphere... Nibbana
Atthi, bhikkhave, tadāyatanaṁ... Nibbana
Ud 8.1
Lastly:
There are these two elements:
Dve imā, ānanda, dhātuyo—
the conditioned element and the unconditioned element.
saṅkhatādhātu, asaṅkhatādhātu.
MN 115
A lot of confusion arises dependent upon treating nibbana as a conceptual mental object dependent upon mind. This ^^^ leads to lots of confusion. An analogy describing this confusion is a man dying of thirst who refuses to drink life-giving water because the man dogmatically believes water is a mental concept rather than a suffering-free reality.