Much like Sankha and Ruben2020 have said, I think it is imperative to understand that selflessness is an apt description of all persons and phenomena, including nirvana. That is, there is nothing which isn't selfless. Another way of saying this is that all things lack inherent existence; including nirvana. There is nothing neither existent nor non-existent that cannot be described as utterly lacking a self.
But nirvana is not a person so how can it be said to be selfless? Also, it is not an impermanent phenomena so how can it be said to be selfless?
One part of the confusion here is that it is usually thought that the Pali Canon is only part of the First Turning of the Wheel of Dharma. However, I don't think this is strictly true. The Second Turning of the Wheel of Dharma can be found in the Pali Canon. In Sutras like this and for instance the Kaccayanagotta Sutta we see the Buddha giving teachings that to my mind are very clearly part of the Second Turning. These teachings are elaborated on by the Buddha in strictly Mayahana Sutras and by the Virtuous Teachers such as Nagarjuna. Nagarjuna's Fundamental Treatise of the Middle Way devotes an entire chapter to nirvana and how it is selfless. So how do the Second Turning Virtuous Teachers say that nirvana is selfless?
Nirvana is not a person and yet it utterly lacks any inherent existence. This is how it is said to be selfless. Nirvana is not an impermanent phenomena and yet it utterly lacks any inherent existence. This is how it is said to be selfless.
Thus, the correct understanding of the selflessness of Nirvana cannot be arrived at without understanding the subtler meanings of selflessness. Understanding selflessness through things not existing permanently, unitarily and independently is the coarsest understanding and this is not sufficient for understanding the selflessness of Nirvana.
Now, does that mean being selfless is a completely sufficient description of nirvana? No. More needs to be said. Nevertheless, nirvana is indeed selfless.
Moreover, understanding the selflessness of all things is essential to gaining our liberation from cyclic existence. Directly realizing the emptiness of all existent and non-existent things and regarding all this as a morass of utterly unreal concepts and appearances is how we arrive at the doorstep of nirvana. I hope this helps.
Finally, I'll note that nirvana comes in two types: with residuals and without. According to my tradition Nirvana without residuals is only attained by fully enlightened Buddhas or Arya beings while in meditative equipoise directly perceiving emptiness.
