Does a creature that is phased out still count toward devotion?

When a permanent is phased out, it is treated as though it does not exist for almost all game purposes (CR 702.99d). This means the game simply ignores it — it is not on the battlefield, it cannot be targeted, and its characteristics, including the mana symbols in its mana cost, are not counted for any continuous effects or conditions that look at permanents on the battlefield.

Devotion, as defined in CR 700.5, counts the number of mana symbols of a specific color among the mana costs of permanents you control. Because a phased-out permanent is treated as though it doesn't exist, it is not considered to be on the battlefield, and therefore you do not control it for game purposes while it is phased out. Its colored mana symbols are invisible to devotion calculations.

This is importantly different from a face-down permanent (which still exists on the battlefield but hides its characteristics) or a tapped permanent (which still exists and contributes normally). Phasing is unique in removing the permanent from the game's awareness entirely until it phases back in.

Concrete example: You control Gray Merchant of Asphodel and a creature with two black mana symbols ({B}{B}) in its cost. That creature phases out. When Gray Merchant's ability triggers, only the {B} from Gray Merchant itself counts — the phased-out creature contributes 0, so your devotion to black drops by 2 compared to when it was present.