Deity is a cultural word, word usage can differ by time, place, culture. I see no issue with using deity for devas. The objection to using deity for Buddhas and Bodhisattvas (in Mahayana, referring to Bhumi 10 Bodhisattvas) would be that it could create confusion that the enlightened ones are just gods, whereas, they are way above gods level. In Theravada, Bodhisattas can be in any realm, many Jataka stories has Bodhisatta as animals, so naturally deity is only applied to Bodhisatta if they are reborn in the deva realms.
From Meriam Webster dictionary:
Definition of deity
1 a**:** the rank or essential nature of a god : DIVINITY
b capitalized : GOD sense 1, SUPREME BEING
2**:** a god (see GOD entry 1 sense 2) or goddessthe deities of ancient Greece
3**:** one exalted or revered as supremely good or powerfulsuch established American deities as Daniel Boone, Kit Carson— J. D. Hartthe deities of the banking world
In using deity for devas, we are using definition: 1a, 2, 3. Since there’s no such thing as 1b in Buddhism. In terms of the understanding of the outside world with regards to gods, we tend to equate devas with the Greek gods, or Egyptian gods, or Norse gods, or angels in Christianity, or the various gods in Chinese folk religion, it’s best to use deity to refer to those, and not downgrade Buddhas to those levels.
Many religions has the aim to be reborn in heaven, become a god, whereas Buddhism goes way beyond that aim to end samsara all together. If you use the term deity for enlightened beings, you’re risking a miscommunication of the ultimate aim of Buddhism for others.
As for devas to attain to enlightenment, here’s some logic.
- Once returner and stream winners are destined to become arahants, they can be reborn in the deva worlds.
- Specifically for once returner, once they got reborn in deva world, in that very life they got to go higher up the enlightenment rank as they only come back to the sensual world once, as their name implies. Thus it’s minimum possible to train to become a non-returner in deva realms.
- Since it’s possible to become non-returner, which requires the cultivation of Jhanas in deva realms, there’s no a priori objection to devas attaining any of the other enlightenment stages, from stream winner to arahant.
- As for Buddhahood, it’s a special attainment which has many lores associated with it. One of the Theravada lore is that Buddhas only appear in human realms. So the Bodhisattas, just before their final birth to become a Buddha are devas in the Tusita heaven, then purposely got reborn as a human then become a Buddha. All Buddhas follow the same pattern. Thus, it’s not possible for devas to become Buddhas, but as Bodhisattas, they can certainly hang out in deva realms while biding their time to appear as the next Buddha.