Install this resource pack :: https://www.planetminecraft.com/texture-pack/vanilla-emissive-ores-6379653/
Hold a gold ore in your hand
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Quads from block and item models always go through VertexConsumer#putBulkData. However, light_emission is only applied right after flat lighting and smooth lighting during block model buffering and ignored during item model buffering. Considering that this feature would have been easier to implement by applying the light value inside of VertexConsumer#putBulkData instead, the fact that items do not support light_emission seems intentional to me. However, I agree that items should support it.
This issue seems to be more generally that item models do not support emissive textures in any form (dropped on the ground, held, or worn).
This may be a feature request. The release article only states block models: "Added support for emissive layers for block models"