Claude Litre, a fictional character created by Kenneth Woolner in 1978, was used to justify the capital L to denote litres. This practice arose due to potential confusion between the lowercase “l” and other characters. Woolner’s hoax was published in “CHEM 13 News” on April Fools’ Day, claiming Litre proposed a unit of volume measurement that was later adopted into the International System of Units. The hoax was mistakenly printed as fact in the IUPAC journal, Chemistry International, and later retracted. The use of the uppercase letter L for litres is now recommended by the United States National Institute of Standards and Technology, as well as widely followed in Canada and Australia.
https://en.wikipedia.org/wiki/Claude_%C3%89mile_Jean-Baptiste_Litre