File Replication Service (FRS) has been deprecated. You will need to update to DFSR before building a new DC in your domain if you are still using FRS to replicate SYSVOL.
FRS and DFSR both asynchronously replicate content sets of file data. FRS was introduced in Server 2000. DFSR was introduced in Server 2003 R2 however gained the ability to replicate SYSVOL on DC’s with Server 2008.
Server 2016 is the last Server OS to support FRS. If you have been running your domain since Server 2003 or 2000 domain functional level you may still be using FRS for replicating SYSVOL.
FRS was actually deprecated since Server 2008 R2 when building a new domain however it supported FRS if you migrated from 2003 or earlier.
With Server 2012 the default domain functional level for new domains was Server 2012, therefore you could not select a functional level that uses FRS when creating a new domain.
What’s the advantage of using DFSR vs FRS ? DFSR replicates on a block-level basis compared to the full file replication.
Have a look at this MS article if you want to go ahead and migrate from FRS to DFSR.
By the way in 2016 MS said you should migrate to DFSR immediately. It won’t actually take long to follow the instructions. I estimate 15 minutes to read through the instructions and follow them.
The forest functional level must be 2008 or higher.
Need more info on this, read this MS article.