I've just been to Switzerland and though I had a letter no one asked to see it.
In 20 years I've never been asked for one, but I was concerned about heightened security & flying with a CGM for the first time.
I just tell them I am diabetic and have insulin and pens in my bag.
My GP charged me £20 for the letter. I suppose it at least it gave me peace of mind in case it was requested.
I do think it's better to have a letter when your travelling abroad, I've not been stopped before and asked to show the letter but the reassurance is there if I did.
You can get a letter from your hospital diabetes team (if under their care) usually for nothing, as DunePlodder has found out GP's can charge for such letters.
I think it's better to have a letter just in case.
It also depends on where you're flying to. I've found that flying out from the UK is fine (I did get stopped at security but that was because I had scissors in my bag...) but some airports on the way back may be more problematic if they haven't seen it before.
I took a letter and my prescription just in case; I've just travelled to America and Canada where their airport security is a lot stricter than ours and I wasn't asked anything about my insulin or epipens and massive box of needles I've been to Majorca as well and again I wasn't asked anything but I would always take it just in case
In 10 years the only place I've been asked for a doctors note was travelling through Malaysia into Singapore but I carry one just in case.
Malaga airport let me take a whole bottle of lucozade through security which I thought was odd (left it in my handbag by mistake) but meant I didn't have to buy a new one lol.