The Immigration Services Agency of Japan has announced the extension of the period of validity for Certificates of Eligibility (in relation to COVID-19) as follows.

The extension is available only in cases where the Certificate of Eligibility was issued on or after 1st October 2019.
・COEs issued between 1st October 2019 and 31st December 2019 will be treated as valid until 30th April 2021.
・COEs issued between 1st January 2020 and 30th January 2021 will be treated as valid until 31st July 2021.
・COEs issued on or after 31st January 2021 will be treated as valid for 6 months from their date of issue.

Refer to the below sites for further details.
http://www.moj.go.jp/isa/content/930005849.pdf

http://www.moj.go.jp/isa/nyuukokukanri01_00155.html

Please note that even with the extension of the period of validity of the COE, a valid visa is still required.
If an applicant has been granted a visa using their COE, but has not been able to enter Japan before the expiry date of the visa due to the COVID-19 outbreak and meets the conditions above, they may re-use their COE to apply for a new visa.