Android 12 Eligibility :
Samsung Galaxy A21, initially released on April 2020 came with One UI 2.0 on top of Android 10. And it got One UI 3.0 on top of Android 11 earlier this year in some markets and currently rolling out in other markets (as of sept 2021). As Samsung Now Updates all devices with Minimum 2 Major Years Update Samsung A21 should be eligible for Android 12 which is very impressive considering the price point of this device. And with Android 12 Samsung devices have its own One UI Skin that also will be updated to One UI 4 with New features and design. And it should be the Last Major Android update.
As Android 12 is still in beta and expected to release this Month (September 2021) or early next month. The Release schedule for Samsung Galaxy A21 is unknown. But we expect the release period to be around April to September 2022 or earlier. Since Samsung is currently releasing Beta version of Android 12 for Samsung S21 series One UI 4 features list will officially release soon.
We will update the article accordingly when the schedule release. You can check update status down here for Major and security update status. Also we will be updating list according to first release. Some regions will be getting update in weeks or months after first release.
Since Samsung did not officially released Android 12 update list so the eligibility could change we will keep updated.
[Update 17-Nov-2021] : According to Samsung List of One ui 4 update schedule. Samsung A21 will get one ui 4 based on Android 12 update in July 2022. The update may vary according to region you can check out One UI Schedule for your region in Samsung Members App.