Does anyone have up-to-date data on "excess deaths" in Japan for the past few months?
Yes. That fits data out thus far.
https://www.cdc.gov/mmwr/volumes/69/wr/mm6912e2.htm
> Although the majority of reported COVID-19 cases in China were mild (81%)
Combine the 19% severe cases number out of China with Japan's aging population (which COVID-19 hits substantially harder), and 1/4 is entirely reasonable.
> The largest cohort of >44,000 persons with COVID-19 from China showed that illness severity can range from mild to critical38:
> * Mild to moderate (mild symptoms up to mild pneumonia): 81%
> * Severe (dyspnea, hypoxia, or >50% lung involvement on imaging): 14%
> * Critical (respiratory failure, shock, or multiorgan system dysfunction): 5%