Excess disinfectant can be harmful to animals, but there is no evidence that pets have died after chewing on disinfected boxes specifically.
Disinfection is an efficient way to kill the pathogenic microorganisms that can infectious diseases. However, the majority of the active components are harmful and corrosive chemical compounds, including chlorine-releasing agents, oxidizing agents, and quaternary ammonium cations, an article by National Center for Biotechnology Information (NCBI) states.
According to National Geographic, in February 2020, the Chongqing Forestry Bureau found that over 100 wild animals, including wild boars, Siberian weasels, and blackbirds, had died after exposure to disinfectants. The National Geographic continues: "China was the first country, in January 2020, to start sanitizing its cities—and as soon as it did, reports of poisoned animals started coming in."
Although excess use of disinfectant does pose a significant risk to animals, there have been no reports to affirm that household pets have been affected and have died in the past year.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19 including symptoms, prevention and available treatment, please refer to the World Health Organisation or your national healthcare authority.