Yes, closets can be used as return air plenums.
The 7th Edition, Florida Mechanical Code, definition of a PLENUM: An enclosed portion of the building structure, other than an occupiable space being conditioned, that is designed to allow air movement, and thereby serve as part of an air distribution system.
The 7th Edition, Florida Mechanical Code, Section 602 requires that materials exposed in a plenum must comply with 25 fire/50 smoke ratings when tested in accordance with ASTM E84. This same Section allows for the use of gypsum board to form plenums, as long as the air temperature does not exceed 125 F, the gypsum board surface temperature be maintained above dew-point temperature and not be incorporated in A/H systems utilizing evaporative coolers.
The 7th Edition, 2020 Florida Energy Conservation Code, Section, C403.2.9.1.1, Insulation required. All supply, return and plenums must be insulated. However, when the supply, return or plenum is located in the conditioned or indirectly conditioned space Table C403.2.9.1 does not require any insulation.
