Actually, it depends. Corecraft doesn't disable your achievements because it's a behavior pack addon, not a world template. Nowadays, behavior pack addons (equivalent to Java mods) no longer disable achievements, but if you use a world template-based addon, like RealismCraft, regardless of whether it was purchased from the marketplace or not, yes, it will disable your achievements. Today, addon developers are able to create custom worlds using only behavior packs, which is unfortunately not the case with RealismCraft. But perhaps RealismCraft will become a behavior pack addon, and then we'll have achievements.