Disable MinecraftCapes support - gets rid of the message, but also this provider's capes in general (you can no longer use the free cape instructions above).
Go to Options... → Skin Customization... → → Toggle → MinecraftCapes: OFF → Done → Done → Done.
Remove Capes and installCosmetica - fixes the message and keeps other providers, but the experience can highly vary from FO defaults, so it is recommended to configure the mod to suit your needs.
Go to Mods → search "Cosmetica" → press → configure by your preferences.
Fabulously Optimized does not host any capes or other cosmetics, nor does it encourage users to buy them from any provider. Instead, the modpack encourages users to prefer free cosmetics, while also giving them options to use any paid cosmetics they've previously obtained from certain providers.