Is fumigated barn wood safe for indoor use?
Most likely it was fumigated with Sulfuryl fluoride

In the United States, most experts performing termite fumigation for residential buildings use a gas with the active ingredient sulfuryl fluoride.
After fumigation, the tent over the treatment area is removed, and the fumigant gases diffuse into the air, rapidly leaving your home. Following six hours of ventilation, your pest management professional will use devices that detect trace amounts of sulfuryl fluoride and chloropicrin in the air. According to the United States Environmental Protection Agency (EPA), re-entry is not permitted until sulfuryl fluoride levels are 1 part per million or less. (During the fumigation, concentrations may reach 3,850 parts per million or higher.)

According to Cornell university, it quickly breaks down

Sulfuryl fluoride dissipates quickly in the atmosphere and is broken down through hydrolysis and photodegradation.

If it's been some time since the fumigation, it's likely safe enough to use indoors