As per the announcement of the Royal Thai Government, the permitted period of temporary stay for foreigners in Thailand will end on 31 October 2020. Any foreign nationals who wish to stay in Thailand beyond that date must submit an application for extension of temporary stay at the Immigration Bureau within 31 October 2020.
Late application for extension of stay can be submitted within 90 days after 31 October, subject to a fine. After the end of such period, those who remain in Thailand or have not applied for extension of temporary stay will be deemed as overstaying. They will subsequently be prosecuted, deported and prohibited from entering into Thailand.
Additional information on this matter can be found on the website of Immigration Bureau of the Kingdom of Thailand: https://www.immigration.go.th/en/