A subscribing witness's testimony is necessary to authenticate a writing only if required by the law of the jurisdiction that governs its validity.
Notes of Advisory Committee on Proposed Rules
The common law required that attesting witnesses be produced or accounted for. Today the requirement has generally been abolished except with respect to documents which must be attested to be valid, e.g. wills in some states. McCormick §188. Uniform Rule 71; California Evidence Code §1411; Kansas Code of Civil Procedure §60–468; New Jersey Evidence Rule 71; New York CPLR Rule 4537.
Committee Notes on Rules—2011 Amendment
The language of Rule 903 has been amended as part of the restyling of the Evidence Rules to make them more easily understood and to make style and terminology consistent throughout the rules. These changes are intended to be stylistic only. There is no intent to change any result in any ruling on evidence admissibility.