Waste paper is an uncountable noun. Oxford Learner's Dictionaries https://www.oxfordlearnersdictionaries.com/us/definition/english/waste-paper?q=waste+paper waste paper noun BrE /ˌweɪst ˈpeɪpə(r)/ ; NAmE /ˌweɪst ˈpeɪpər/ [uncountable] ​paper that is not wanted and is thrown away --------------------- However, "littered with paper" and "littered with papers" are both natural. The latter emphasizes individual sheets of paper. The term "waste papers" appears to be a correct but uncommon term that refers to individual sheets of discarded or used paper. - There are papers on the floor. (= There are sheets of paper on the floor.) - There are waste papers on the floor. (= There are discarded sheets of paper on the floor.) Here is an Ngram for waste papers, discarded papers, and used papers. https://books.google.com/ngrams/graph?content=waste+papers%2Cdiscarded+papers%2C+used+papers&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cwaste%20papers%3B%2Cc0%3B.t1%3B%2Cdiscarded%20papers%3B%2Cc0%3B.t1%3B%2Cused%20papers%3B%2Cc0

Actually it should be "The floor was littered with paper." Doesn't matter if it is waste or not.

You are correct, delete the ‘s’, “waste paper” is correct here. Think of a ‘waste paper basket’ (a container/bin for rubbish often found in offices to dispose of used paper), to remember this.