For nearly a year,  many so-called yellow shops in Hong Kong allowed customers who backed anti-government protests in the city to write messages of support on Post-it notes and stick them on the walls of the establishments. But since June 2020, when Chinese mainland authorities imposed a national security law on the city, some restaurants and stores have cleared their so-called Lennon walls. The owner of one “yellow” cafe in the city told the Post about what he sees as the intangible impact of…