A common explanation given for the lack of excess mortality in places like Japan is masking; however, there are many places that had low masking rates and low excess mortality rates during the spring 2020 wave.
For example, Denmark, Norway, and Australia all had low masking rates and low excess mortality rates.
Australia is particularly notable as it's a top destination for Chinese travelers.
Singapore was late to adopt masking and is also a top destination of Chinese travelers.
"For weeks after the first reports of a mysterious new virus in Wuhan, millions of people poured out of the central Chinese city, cramming onto buses, trains and planes as the first wave of China's great Lunar New Year migration broke across the nation....
...Officials finally began to seal the borders on Jan. 23. But it was too late.
Speaking to reporters a few days after the city was put under quarantine, the mayor estimated that 5 million people had already left...
...The top 10 global destinations for travelers from high-risk Chinese cities around Lunar New Year, according to their analysis, were Thailand, Japan, Hong Kong, Taiwan, South Korea, the United States, Malaysia, Singapore, Vietnam and Australia."
https://www.coviddatahub.com/engagement-with-recommended-behaviours
https://ourworldindata.org/grapher/excess-deaths-daily-per-100k-economist
https://www.nytimes.com/interactive/2020/03/22/world/coronavirus-spread.html