Anything easily applied, like a pin, can be easily removed. Human nature being what it is, I stopped adorning my rucksack with pins bought along the way.
I was losing too many souvenirs at cafe stops. Some of the "misappropriations" damaged my bag when pins were forcibly removed that actually had horizontal pins, not just a post and fastener. Now, they ride in a small nylon bag INSIDE. NO, I will not call this theft. Whatever, it is not important and it is easily adjusted for.
I recommend anything sewn on. However, I suggest you only sew patches, or do embroidery, on a portion of the bag that will NOT be exposed to rainfall or other precipitation. The tiny holes made by the needle will admit water into what WAS a water-resistant rucksack. Of course, you could just use seam sealer on the inside of the bag, where the thread penetrates the bag...
I had my local cobbler (shoe maker / repair guy) sew all my patches on. They usually have a "free arm" sewing machine that normally is used INSIDE a shoe. So, it works very well with bunched up pack material. Plus the result is super strong. the only issue is whether the needle can puncture your material. This was tested and found to not be a problem. Evidently, my fellow had previously tried to sew patches on military grade ballistic nylon bags and broke needles. I assured him that our rucksacks were much lighter material.
I sport my Forum patches, APOC patches, CSJ patch, and several others I picked up along the way. My bag is never misidentified. Before I did this it actually was. I used an Osprey Kestrel 48 liter bag in Conifer Green for my first three Caminos. Evidently, this is a very popular bag and color.
At the very least, I would recommend a silk scarf or some other "flag" like a piece of brightly colored yarn or cloth tied to the outside top of the rucksack like a pennant.
Drying laundry pinned to your rucksack will also distinguish your from the others...
I hope this helps.