It’s my understanding that this is common practice among the top-tier groups (NOP (former), BTC). Altitude camps as an excuse/rationalization for iron injections. Not a banned substance (method may need justification?), but stinks of being against the spirit of the sport.
I know people who had consistently low iron despite taking an oral supplement and had substantial boosts in hemoglobin after going to a injections.
And I think the practice was/is widespread in Japan HS, with a few cases causing hospitalization and getting them to act on it.