Will 2 x 10 speed front derailleur work with 11 speed set up? Would an existing 2x10 front mech work with a new 11 speed rear set up? Using all Shimano SLX with 11 speed shifter, rear mech and chain. Front chain rings are 2 x SLX 38/28.
11 and 10 speed cranksets have slightly different chainring spacings to accommodate the difference in 11 and 10 speed chain widths. Front derailleurs have slightly different movement also.

You have a matching 10 speed front derailleur and crankset, so really the question is whether an 11 speed chain will work on a 10 speed front. I think from past answers I have seen here it will work OK.