I remember in past years migrations that using the /mir switch with Robocopy was generally not recommended due to the fact that if files are removed in the destination directory, they'll also be removed in the source directory. I was only bitten by that once in actual practice.
I am wondering what switches are currently best practice to use instead to get the same content copied but remove the risk of a deletion on destination propagating back to the source folder.
A backup will solve that.
You may want to use this as this also copies ownership of Redirected folders
Robocopy D:\Source \\Target /E /SECFIX /SEC /XO /XJ /ZB /COPYALL /MIR /DCOPY:T /R:0 /W:0 /NP /NFL /NDL /TEE /LOG:"C:\Robocopy.log"