Does Samsung Galaxy Fold SM-F900U support t-mobile band 71?

Yes, the Samsung Galaxy Fold SM-F900U (FCC ID: A3LSMF900U) supports T-mobile band 71

and here is the Band & Mode supported by the Galaxy Fold SM-F900U model.

CDMA/EVDO BC10 (§90S)
CDMA/EVDO BC0 (§22H)
PCS CDMA/EVDO
GSM/GPRS/EDGE 850
GSM/GPRS/EDGE 1900
UMTS 850
UMTS 1750
UMTS 1900
LTE Band 71
LTE Band 12
LTE Band 13
LTE Band 14
LTE Band 26 (Cell)
LTE Band 5 (Cell)
LTE Band 66 (AWS)
LTE Band 4 (AWS)
LTE Band 25 (PCS)
LTE Band 2 (PCS)
LTE Band 30
LTE Band 7
LTE Band 41

To check the Samsung Galaxy Fold SM-F900U Frequency Bands and Network Compatibility, please refer to Samsung Galaxy Fold SAR test report

RF-Exposure-Info-1-4389721.pdf

Samsung Galaxy Fold SM-F900U FCC id: A3LSMF900U

If want to check the Galaxy Fold specs in detail, please visit Samsung USA official website here.

and here you can download the digital Galaxy Fold SM-F900U User Guide in English in PDF format.