The idea that simple serotonin deficiency is the whole entirety of all depressions is 100% discredited and completely incoherent in the face of current evidence, but yes, for sure it remains clear and plausible that some forms or aspects of depression involve a serotonin deficiency.

However, tianeptine is serotonin reuptake enhancer and also can help with depression, so any simple deficiency hypothesis also doesn't look good either.

Broadly, the "chemical imbalance" theory, left vague and unspecified, is still basically sane (though not specific enough to be super useful).