Was there a debate in the Talmud regarding weather Klal Yisroel was still mechuyav Mitzvoth in Galus after the Churbun?
No. The conversation of the Gemara takes place after the churban. The fact that they are discussing the practical application of halacha proves they held ot binding in their era.
I think that I found this talmudical debate the words are not precisely the word of the OP, but the consequences are the same. Up to day I have not found a better source.
This Gemara is in Eruvin 64b-65a:
R'Shesheth citing R'Eleazar B`Azariah observed: I could justify the exemption from judgment of all the [Israelite] world since the day of the destruction of the Temple until the present time, for it is said in Scripture: Therefore hear now this, thou afflicted and drunken but not with wine.
The Gemara comes after a discussion about the capacity of a Chacham to think rightly when he is drunk.
Later, the gemara shows an adverse opinion which debates with this. And din is discussed as din Tefila, and two kinds of drunkennes are separated...
לטעון על כל ישראל לפני הקב''ה דבר שיפטרו כולן מדינא לע''ל לומר שהן שיכורים ואין נתפשין על עונם: To advocate for the whole Israeli (Jew) people, before the Holy Blessed Be He, an argument which can exonerate everybody from the forthcoming judgment, saying that they are drunk and do not be caught because their fault.