I tried some further searches: Google didn't crawl the subdomain /en/radiation-maps before 2022, nor did the see the string "saveecobot.com/en/radiation-maps" anywhere on the internet.
Their twitter account is 5 years old and has a screenshot showing they launched a radiation map 2020. https://twitter.com/SaveEcoBot/status/1339958096232329216/ph... So it didn't appear recently, maybe it was not popular enough to be indexed or linked to since launch (only 22 Twitter followers).
Just because it wasn't in the Archive.org's archive doesn't mean it didn't exist. Many websites are not indexed until someone links to them (shocker), and since no one really cared too much about Chernobyl until yesterday/today, it's not that weird that they (Archive.org) didn't come across the website until today.
