Is it safe to use LiFe PO4 batteries in a 18650 Battery Shield (sometimes called Lithium-Battery Shield)?
In my opinion it is NOT SAFE, if your 18650 Battery Shield also charges the battery.
Anyhow some pages state that it is possible to charge LiFePo4 batteries with 4.2V instead of the recommended 3.65V. https://www.powerstream.com/lithium-phosphate-charge-voltage.htm
LiFePo4 batteries have a smaller energy density and therefore capacity. Thus those are not as explosive as Li-Ion batteries. But you are risking damaging the battery and shorten its lifespan.