We don't have synonyms for indispemsible yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are indispensable and indispensible.
°(ecclesiastical obsolete) Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules.
°Of rules, duties etc: unbendable, that cannot be set aside or ignored.
°(ecclesiastical obsolete) Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules.
°Of rules, duties etc: unbendable, that cannot be set aside or ignored.