Being blue on Twitter was once considered a person's status was important in the community. But since it can be purchased through X Premium, some are reluctant to display this cover because it is seen as a person who pays to look important. So last year X allowed X Premium customers to hide this smear. However this feature may be discontinued in the near future.
According to Nima Owji's comment above, X started giving notifications that this smear hiding feature will be discontinued in the near future. It appears in X Premium client settings. The feature looks set to be removed after X began giving free blue smears to accounts with more than 2500 followers who have verified their identity this week.
Under Elon Musk's ownership, several features were introduced before being discontinued. Voice calls for example were given last year but the Twitter Circles feature was turned off even though it was quite useful. Musk also said that later he will turn off the "Block" feature that prevents some accounts from reading tweets sent.