Does the carrier matter when flashing custom roms? I am getting the lg g6 tomorrow and i saw that there was twrp and lineage and a couple others. But xda also has a thread that is specifically tmobile and thats where im getting my phone. but the tmobile thread had 0 developement going on. So will the carrier matter?
Yes and no. Most times probably not, but there are some carriers which not only apply software modifications but also obtain specific hardware configurations from manufacturers (AFAIK Verizon is one of those having done that in the past for some devices at least, and T-Mobile was mentioned in that context as well).

Which means, there's no "generic answer" to this question, but you must check for each device separately. If the hardware matches, the carrier should be irrelevant concerning the custom ROM – as the latter then again matches the hardware.