Should I avoid saturated fat, trans fat, butter, or coconut oil?
Industrial trans fat is the clearest "avoid or minimize" category. In the United States, partially hydrogenated oils are no longer generally recognized as safe for use in food, but checking labels is still reasonable for some packaged, fried, or shelf-stable foods. Naturally occurring trans fats in small amounts from ruminant foods are a different context, but they are usually not the main issue in normal diets.
Saturated fat is different. It is not poison, and it is not morally bad. Many normal, healthy whole foods contain saturated fat, and saturated fat and cholesterol participate in ordinary cellular and hormone physiology. The question is not "Is saturated fat evil?". The better question is "How much am I eating, from what foods, replacing what, and what is happening to my bloodwork?".
The main practical concern is LDL cholesterol. Higher saturated-fat intake raises LDL in many people, and LDL is an important cardiovascular-risk marker. But the effect is not identical in everyone, and the outcome depends on the whole diet, body composition, activity, genetics, fiber intake, and food quality. Replacing saturated fat with unsaturated fat is generally more favorable than replacing it with refined carbohydrate.
Within appropriate calories, high food quality, good body composition, regular activity, and favorable lipid markers, saturated fat is unlikely to deserve the kind of fear it often receives. But that does not mean saturated fat is irrelevant. Some people are strong LDL responders, and for them, butter, coconut oil, fatty meats, cheese, cream, and large amounts of high-saturated-fat foods may matter a lot. A lipid panel is more useful than ideology.
Butter and coconut oil can fit if you like them and portions are controlled. They are not special fat-loss foods, even in the context of keto or Paleo diets. Coconut oil is mostly saturated fat, and the fact that it contains some medium-chain triglycerides does not make unlimited coconut oil a metabolic loophole. Butter is delicious, but it is still calorie-dense and saturated-fat-rich.
The practical default is measured: Minimize trans fat, keep saturated fat in perspective, use more unsaturated fats as the everyday default, and let health markers guide how strict you need to be. If your LDL cholesterol, ApoB, blood pressure, body composition, and overall diet are excellent, the occasional buttered steak or coconut curry is not the thing to panic about. If your markers are moving the wrong direction, saturated-fat sources are worth considering.