Both times I renewed my passport the old ones had the corner snipped.
I've never heard of it not being done, but maybe they only snip if the old passport still has remaining validity (both mine were renewed way in advance of the supposed renewal date)?
My old passport had not expired, but was full. At Trendy they did not ask me to hand it over for them to cut the corner off. (I had two valid UK passports at the time, the renewed one made three).