Are plastic (vs metal) immersion blenders safe for hot mixtures?

Short of the danger of splashing hot liquid on you, or the standard immersion blender dangers of having it slip and hit you, there's no additional risk with using immersion blenders on hot mixtures. Making hot soups creamy is pretty much exactly what they are designed for.

I'm afraid I don't that hard evidence to back up the following statements, but I thought I'd offer my thoughts based on the clarifications to the question from the OP's comments.

1. Regarding BPA/chemicals leaching into food: This is likely a concern for most plastic products produced before a year or two ago (and a good number of them that have been produced recently, too). However, I would say that in the grand scheme of health hazards related to cooking gear and food, your plastic immersion blender is mostly on the "Don't worry about it" end. You aren't storing food in this object, just putting it into contact with your food for a few seconds to a few minutes. Yes, if the food is hot, there will probably be some leaching of chemicals, but it should be minuscule.
2. Regarding the possibility of the plastic melting: Again, I don't really have an evidence to back this up, but I'd say you're pretty safe on this front, too. Braun is an established and respected brand, so they likely test their products pretty vigorously before sending them to market. It's hard for me to imagine them producing an immersion blender that is not safe to use with hot foods. Additionally, if you've been using the immersion blender for 10 years and haven't had an issues yet, you're probably more likely to have the motor wear out than to have to deal with melting plastic.

So a note on this that no one has discussed, it’s not always about the housing. I’ve been through two metal immersion blenders that I was assured were fine for hot foods and both were ruined due to the lubricant in the blade mechanism not being suitable for heat. This meant the oil leached out and the mechanics ceased. Before that happened, food started to taste chemical metallic, and I noticed after cleaning a dark liquid would appear on the surface I had it placed on where it was draining/drying. Spend the money, get a pro immersion blender. I’m still looking for the best one. My suggestion in the interim with plastic or metal is to plan ahead so you can allow your foods to cool first. Then re-heat food before serving.

My blender just melted (distorted shape) in hot soup and the blade took lumps out of the bottom of the metal pot. Had to throw the whole batch away.