Hi @Jammy97 yes Android 13 will be available for the S21 series, they are amongst the devices eligible for 4 cycles of OS updates. see more here https://www.androidpolice.com/samsung-phone-list-four-years-updates/
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 4.0,Android 12 Official Version.
Most definitely however the timeframe i.e when a phone / region is yet to be announced once any Beta Testing has been concluded @Jammy97
Daily Driver > Samsung Galaxy s²² Ultra 256Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I have no affiliation with Samsung.