The bug

The item form of air does not appear to be able to have its item model changed under any circumstances. This is inconsistent with air's block form; while its physical model cannot be changed due to it being hardcoded to be invisible, its particle texture can in fact be modified.

As a result of this bug, MC-122177 is impossible to fix even with the use of resource packs, even though MC-239407 can in fact be fixed.

How to reproduce

A resource pack is attached to this ticket in order to make this issue clearer to see. A dedicated debug texture for air is added, and two identical model files are added for air's block form and air's item form. You are encouraged to check these model files to verify that this is true.

1. Download and apply the attached resource pack

2. /particle minecraft:block_marker minecraft:air

3. /particle minecraft:item minecraft:air

Expected results

With the resource pack applied, both of the commands above would produce the red-and-white texture included in the resource pack.

Actual results

Only the block_marker command produces the expected texture. Despite the resource pack very explicitly specifying otherwise, the air item still unaccountably produces a missing texture particle.

Further notes

If this issue is fixed, it will be trivial to completely eliminate the missing texture from appearing in the default resource pack. It is already possible to fix MC-206684 and MC-239407 with resource packs, and fixing this issue would also make MC-122177 very easily fixable via similar means. If this is done, the missing texture will never appear in the vanilla resource pack at all, making it the first version of Java Edition since 14w04b for this to be the case, since every version since then has had it appear in some form. The texture would still, of course, appear in broken resource packs.