At my company, we had someone who told us they were planning on moving countries within a year, but hadn’t given a firm date to leave. We interpreted it as we weren’t given an official notice until they confirmed the date.
Even after they gave a date, we still treated them as a normal employee, but we did tell them we expected that their quality of work wouldn’t drop during this period. I don’t know how that would work in positions where the policy is to cut off access after someone gives their notice.