This construction in Latin is called the Passive Periphrastic and is one way Roman authors accomplished the idea of necessity. It has basically two parts, the gerundive or future passive participle plus a form of the verb to be. The gerundive declines like a first/second declension adjectives which is why the ending is "um" and not "em" as a third declension would be.Since the participle is an adjective it needs to agree with the subject. In this case the subject is impersonal and omitted therefore neuter nominative. The doer is typically expressed by a dative of agent, omitted in this case. Should the subject have been "taste" the form would have been disputandus since the noun in Latin is masculine, fourth declension.A literal translation is awkward but the idea here is "there ought to be no dispute regarding tastes (personal)" and the idea is more likely aligned with the English sentiment, "to each their own". You're more likely to find the est following disputandum, at least in prose. You may see the est before disputandum in poetry to fit meter.