https://www.theguardian.com/technology/2023/apr/20/twitter-blue-checks-disappear-legacy-accounts-elon-musk

Checkmark denoted accounts verified for authenticity, which can now be bought for an $8 a month subscription called Twitter Blue

Blue checkmarks began to disappear from formerly verified Twitter accounts on Thursday, as policies implemented under new owner Elon Musk began to take hold.