I know this topic has come up before, but I think my question is a little different.
Robert Bringhurst says the default Th ligature in many Adobe OpenType fonts is "misclassified as basic". In order to get InDesign to apply all basic ligatures but not Th, one has to either edit every single Adobe OpenType font using a tool such as FontLab or use some sort of script to "undo" all Th ligatures but not the other basic ones.
I can write a little script, of course, but maybe somebody already has one, or can offer another simple solution to the problem (besides find/change)?