Hi there, thank you for reaching out and sorry for the extended reply time.
The relationships table can be accessed by inspecting the mapImageSublayers property of a AGSArcGISMapImageLayer. Each sublayer will have sublayer info that contains a set of relationship infos.
I'll attach a modified version of the view controller that prototypes this out. It will print the following:
Loading MapServer...
Loaded
BLD_Res_Bld_Type_2020_DL
Est_Bldg_Type_2020_DL
Pop_Sex_2020_DL
Pop_Above15_Less15_2020_DL
HousingUnit_Density_2020_DL
Pop_NEcoRel2WrkFrce2020_DL
Pop_Abv10_Edu_Lev_2020_DL
BLD_BuildingStorey_2020_DL
Pop_Occupation_2020_DL
HousingUnit_Use_2020_DL
Est_by_Sector_2020_DL
Pop_Abv15MF_EcNEco_2020_DL
HousingUnit_Type_2020_DL
Pop_Sector_2020_DL
HHold_Indi_FamSze_2020_DL
Pop_AgeGroups_2020_DL
I hope this helps!