Is Nuclear Spin Isomer a kind of allotrope?
Some peer review journal articles and an India high school textbook and various study guides do refer to the spin isomers as allotropes.  

There are physical differences such as different heat capacities.  See Orthohydrogen, Parahydrogen and Heavy Hydrogen for comprehensive information.  

However, aside from the spin isomers, there are genuine allotropes of hydrogen.  Solid hydrogen has two normal pressure allotropes, a face centered cubic and a hexagonal closest packed allotrope.  

Also, at high pressure, there is metallic hydrogen.