Unfortunately, I could not assure you it is absolutely safe even you use Office 2010 offline in future.
After Microsoft stopped supporting, there is no security updates or bug fixes released for Office 2010. Not sure if there is any unknown loopholes of Office 2010 discovered in future and be exploited to create threats.
Although you only open Office 2010 when offline, some threats have the opportunity to invade your computer if you connect to the Internet to download files, open some links or do other regular use, these threats may be detrimental to your Office 2010 program or Office files.
So a kind reminder, please upgrade your Office product when reaching the end of lifecycle of Office 2010.
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