Will Keloid Injection Affect Breastfeeding?

No. Most importantly, even if the dermatologist uses the strongest (least dilution) strength of steroid 10 mg/cc he or she is likely to be injecting well less than a 1/2 cc. This comes out to 5 mg. tops. Plus this is a depot shot which means that it will be slowly released through time. Thus at any time you will be receiving maybe 2 mg.tops systemically. It would probably take about 20 times that for any effect and probably 40 times that for a deleterious effect on your baby.
Congratulations.