I remember the scene well because you can see Kurt is holding it in his mouth ready and they keep the same studio audience laughter response to it but I've never see him actually vomit. I'm also not good with that kind of thing fake or real so don't mind that it's missing and in 1975 it was probably a risky move to broadcast something like that. Or maybe the take didn't go as planned but they couldn't re-shoot it so it was cut.
As for animal safety on TV even American Humane has been criticised for failing to ensure animal safety on some film sets and still give certification. The law is that animals should be treated with the same safety consideration as human actors but they are still being killed on film set from accidents or failure to ensure their welfare.
I have the Kes DVD and I can't watch it because of the ending but I like the powerful story and the humorous moments with Brian Glover. I saw the play around 1987 and while it was still sad it was an uplifting experience but the theatre is more involved than television. I still have the theme tune cassette that I bought in the foyer.