I posted this in the Discord but thought I could give a more cohesive info about it.
In short in 1.21.9+, with the new EMF/ETF versions the Reimagined skeletons are completely borked where they appear fully invisible.

I'd imagine it has something to do with the Skeleton's using multiple different seperate textures for the same model for their variation system. 1 texture for the skeleton itself, 1 for the clothing & 1 for the equipment/backpack.

Reimagined v1.53.3 resource pack (link)

In 1.21.8:

In 1.21.10 (EMF + ETF)

In 1.21.10 (Only ETF)

Without EMF and only ETF present the texture seems to be displayed, just incorrectly since it's missing it's model.

Broken version test with:

• Minecraft Version: 1.21.10
• Reimagined v1.53.3
• EMF version: 3.0.3
• ETF version: 7.0.4