There are some instances where player heads still query sessionserver.mojang.com:443 once per ItemStack when B:useMojang=false

• Loading the world when a player head item is dropped on the ground
• Loading the hotbar with a player head in it
• Loading inventory with a player head in it
• Moving a player head in inventory

Player heads placed as tile entities do not have this problem. It's a problem for me because my client will hang for a second or more waiting for Mojang when I play offline, but eventually it loads the skin properly through the mod.

I'm using 1.12.2-v6 and haven't tested other MC versions.

Upon creation of the ItemStack for Skull, vanilla code will query session server for respective profile if it has a String type SkullOwner tag. This process only happen once per ItemStack unless the behavior itself got modified.
There is probably mods/utilities/websites that can pre-load profiles in ItemStack and generate a command for you.