I check so many keywords/domains end with S are unavailable. however, I check them in Google trends that the search volume of NO S higher than with S. what do you think ?
S is one of the High tier character And may be the other reason is most of the plurals contain 's' ??
It depends on the word, in my opinion, but many words sound better in their plural form. I like Forums better than Forum, Domains better than Domain, etc. More brandable and sounds better. Like I said, this varies depending on the word combination.
plus the S domains will pick up search engine traffic for the singular and plural. So it's like a double whammy!