Spectator menu does no longer show skins (up to 1.12.2)

Risk of confusion

This report describes this bug only for 1.12.2 (and earlier versions), which has been caused by the removal of the skins.minecraft.net/MinecraftSkins/ API, see WEB-985.

See MC-123447 for the same bug in 1.13(+), which appears to be unrelated to the cause described in this report since it appeared before the removal of the API.

The bug

The spectator menu no longer show skins.

The reason for this bug is WEB-985 since this URL is used by net.minecraft.client.entity.AbstractClientPlayer.getDownloadImageSkin(ResourceLocation, String).

Note: Realms appears to be not affected despite the existence of net.minecraft.realms.RealmsScreen.bindFace(String, String) since it has its own method to download the skins in 1.10.18 (com.mojang.realmsclient.util.RealmsTextureManager.bindFace(final String)).