The presence of a flapper on a vent seems counterintuitive, as restrictions are typically avoided. Could you provide the standard that specifies this?

There is no standard which specifically specifies the use of a flapper. A properly designed flapper should provide de minimus restriction to vent flow, yet still provides weather protection which allows for a vertical release of the vent stack flow, which is best from a dispersion and radiation perspective. Flappers are extensively used successfully and safely on nearly all liquid hydrogen road tankers as well as a supplemental means to vent large flows from stationary tanks in a vertical direction. However, vertical vent systems with a flapper must be monitored to ensure the flapper closes after operation to ensure no water enters the vent systems. LH2 systems are especially susceptible to this due to water freezing on the vent stack cap and its components and expansion contraction.