Are whistles or 6 hole bamboo flutes very popular in this key and if so what other key in music can these be played in other than E if an occasion arises?
The question is a relative one. I can only answer it in terms of playing British and Irish traditional music. For that the answer is no, E isn’t a popular key for a tin whistle.
The fact that an E whistle plays easily in A (in the same way as a D whistle easily plays in G) may be attractive to some. But, really, the tunes that we tend to play in A fit well upon the D whistle because of where they are pitched.
If you play in a band, then having whistles in “odd” keys may be useful. You might be able to accompany a singer or, say, play a tune or a break in an unusual minor key.
If you are playing alone, then I suppose you might be someone who likes the higher pitch of an E whistle, but you’d find it of limited (if any) use in traditional music playing circles (eg. sessions).
There are very few manufacturers of E whistles. I seem to remember that Dixon produced one in their “Trad” range, but I can’t find it on their website at the moment. Though they have only just restarted manufacture over the Covid crisis.
To me, having a Low E whistle is very handy indeed.
Because there are a number of tunes that fiddlers prefer playing in A, that whistleplayers, fluteplayers, and uilleann pipers prefer playing in G.
When a fiddler starts into playing endless tunes in A Major I grab my E whistle and I’m good.
There are various reasons that A Major is more facile on an E whistle, for me the main one is to be able to easily and fully ornament the note C#.
And you have G# right there under your fingers.
The Poster Child for this is The Mason’s Apron which is so very easy to play on an E whistle.
Then, too, I sometimes play with a box player who does like to play certain tunes in E Major.