Learn Russian: Мы написали пресс-релиз, так как новый план уже разработан. - We wrote a press release, as the new plan has already been developed.

Short-form passive participles like разработан belong to a more formal, professional, or written register. Since the context here is corporate or bureaucratic (writing a пресс-релиз), the formal passive is a perfect fit. Using уже разработали (they already developed) would mean the exact same thing but sound much more conversational.

How does Russian distinguish between the present and past passive here, since the English says 'has been developed'?

By the absence or presence of the past-tense verb был. Because there is no был before разработан, the sentence technically says the plan is currently in a developed state. Perfective short-form participles focus on the present result of a completed past action, which translates perfectly to the English present perfect.

Why doesn't this sentence use the reflexive passive form, like разрабатывается?

Reflexive verbs ending in -ся are only used for imperfective passives, describing actions that are ongoing or repeated (for example, 'the plan is being developed'). Because the development of the plan is completely finished, you need the perfective aspect, which requires the short-form participle разработан instead.

What determines the ending of разработан?