On May 1, 2022, Amazon shut down Alexa Internet which subsequently discontinued Alexa Rank with it. While Alexa Internet didn't publicly state why they retired the service, it's likely because of the lack of interest from digital marketers.
Alexa rank is determined using some proprietary methodology that combines average unique daily visits and pageviews per visitor over the past 3 months. Alexa estimates the number of unique daily visitors from the number of users of their global panel who visited a site that day.