/z/ occurs after a voiced sound, so we find dogs /dɒgz/ and homes /həʊmz/. But /s/ occurs after voiceless sounds, resulting in cats /kæts/ and cakes /keɪks/.
Signwriters in Singapore have discovered the use of /z/ as the spelling of the plural 's', but then they tend to use it in all cases, without considering whether the suffix is actually pronounced as /z/ or not.
First, let's look at a sign where the /z/ is phonetically correct, as bag ends with /ɡ/, which is voiced:
However, the next one (on the outside of an ice-cream parlour) is not quite right, as scoop ends with /p/, which is voiceless, so the final 's' should be /s/:
And similarly, have a look at the following sign for a hairdresser's, where the plural of cut should really end with /s/, not /z/:
I guess it doesn't matter too much. The signs are designed to be creative and eye-catching, and the writers are not trying to take a phonetics exam. Maybe we should just celebrate the opportunity for students of linguistics to test their knowledge and see which signs get it right!
(My thanks, once again, to Ludwig Tan for sending me these photos.)
