What is the purpose of plastic insert/plug in Deore FD-M590?
You can search Shimano's Technical Documents site for manuals.

Here's the manual for the M590 BB, crank and front derailleur: https://si.shimano.com/pdfs/si/SI-6SA0A-001-ENG.pdf

The red insert is apparently a 'Pro-Set Alignment Block' and is there to help set the height of the cage above the large chainring.