By beating tons of Galaxy devices out there, Galaxy M21 has received new December Security patch as its Bianual update schedule. It is beat surprise as some new high-end & some mid-range Galaxy A series smartphones have to get new patch yet. The older Galaxy M21 grabs M215FXXS3CWK1 firmware version with 171MB file size.
The changelog only mentions it has improved device security as Samsung promised to tackle over 75 vulnerabilities. You may see some enhanced features on your device. This smartphone is not in the list of One UI 6 software list.