Zevachim 45a - Hilchisa L'mishicha

The gemara paskens like R. Yossi who says that both the machshava and action being done at the time of machshava need to be either outside, or both inside. The gemara wonders why we would pasken a halacha for the time of moshiach since nowadays we don't have a beis hamikdash and have no ability to bring korbanos (some meforshim ask that according to the Rambam korbanos can be brought nowadays because the kedusha of yerushalayim is there, so it is not a halacha for moshiach - here). The gemara concludes that the issues are certainly worthy of discussion - דרוש וקבל שכר, just that we don't need to pasken something which isn't relevant now. See Tosafos for a discussion when we consider something a halacha for moshiach - perhaps only when it is the result of doing something wrong i.e. pigul AND not relevant nowadays.

One way or the other, the discussion is definitely worthy of having - דרוש וקבל שכר. The Mitzpeh Eisan raises an interesting question from the gemara in Yoma 5b regarding how the kohein gadol would get dressed in the future, and the gemara comments מאי דהוה הוה - what was was, meaning that even the discussion isn't relevant nowadays. The gemara only considers the discussion relevant because it helps explain pesukim, but otherwise the gemara accepts that we don't need to discuss what isn't practical. Why doesn't the gemara say that we discuss even what isn't practical because דרוש וקבל שכר?

The Mitzpeh Eisan suggests that this concept of דרוש וקבל שכר only applies to korbanos where the learning about them atones as if we actually fulfilled them. But in other areas that aren't applicable nowadays such as the dressing of the kohanim, we wouldn't discuss it unless it is important for the understanding of pesukim. This idea is supported by rashi at the end of baba metzia 114b with the story of eliyahu hanavi in the cemetery who explains that 4 out of the 6 sedarim of mishnayos is practical - surprisingly listing zeraim among the 2 impractical, and kodshim among the practical since the study of it offers atonement. The difficulty with this approach is that the source of this concept is the gemara at the end of menachos 110a which says that those who study the "halachos" of the korbanos receive atonement as if they actually sacrificed them. It seems like this concept can only be achieved by learning kodshim as if it were halacha l'ma'aseh and knowing who it is we pasken like.