The G20 Osaka Summit and Finance Ministers Meeting will be held in June 2019.
During the G20 Osaka Summit and Finance Ministers Meeting (including several days before and after these meetings), locker services may be closed off to strengthen the security of the railway stations.

Schedule of the G20 Osaka Summit and Finance Ministers Meeting:
June 8th and 9th Finance Ministers Meeting (in Fukuoka City)
June 28th and 29th G20 Osaka Summit (in Osaka City)

Train stations that may not have available locker services.
Shinkansen and conventional railway stations throughout Japan.
(Including railway stations other than the conference venue.)

Period when you may not be able to use the coin locker:
From a few days (roughly one week) before, to the following day of the G20 Osaka Summit and Finance Ministers Meeting.

Baggage Room service may be available in the stations.

Please check below link for more details,

https://www.japan.travel/en/news/g20-osaka-summit-locker-services/