What was a significant milestone reported by the TSA regarding air travel during the pandemic?

The significant milestone reported by the TSA regarding air travel during the pandemic was the screening of over 1 million passengers in one day. This was an important indicator of the gradual recovery in the travel industry as countries began to ease restrictions and more people felt comfortable flying again. Reaching this benchmark suggested that confidence was returning among travelers, which is crucial for the aviation sector's revival.

The other options depict scenarios that did not take place or had varying levels of impact or implementation. For instance, while vaccination passports were discussed, they were not universally adopted as a significant milestone by the TSA. The cancellation of all domestic flights was not a true reflection of the situation during the pandemic, as flight operations continued, albeit at reduced levels. The launch of new airlines was not as prominent a milestone as the screening numbers, since most airlines were focusing on maintaining their existing operations rather than starting new ones during this challenging time.