On this article published yesterday https://help.minecraft.net/hc/en-us/articles/4408894664461-Minecraft-Java-Edition-Skins, and it says "Skin changes in Minecraft version 1.7.9 and later should happen immediately. If you are playing version 1.7.8 or an earlier version, skin changes may take up to an hour to be applied"

So, when you upload a classic skin (which is the original format before 1.8 was released, and you play 1.7.9 and later, everything is fine, but when you play Minecraft 1.7.8 or any earlier, the skin just doesn't work, the default Steve skin appears

I know some older versions of Minecraft can no longer have skins but it clearly says "If you are playing version 1.3 or earlier, skin changes will not be reflected in-game"

And skins between Minecraft 1.3.1 to 1.7.8 should be compatible

And this problem happens to many people :

https://www.reddit.com/r/Minecraft/comments/855ari/playing_on_a_server_on_172_and_no_one_has_skins/

https://www.minecraftforum.net/forums/minecraft-java-edition/discussion/3054634-missing-skin-in-older-versions-of-minecraft

MCL-19983