Treecapitator is one of those mods that people love or hate, which throughout its lifetime has been in popular packs then vanishes for a time. Like it was in the old Technic packs, then wasn't in Tekkit itself for a long while, but then it was back in their later packs like Tekkit Lite. I don't know if it was ever in FTB, now that I think about it. It's one of those mods that grew on me, and almost entirely due to the large vanilla trees which are a pain to chop down. I know that there's a few mod axes to help with that these days, particularly Tinkers, but that locks you into a particular tool path, since most of those require fully investing in their particular mods to make.
Mo' Creatures is another old Technic Pack mod that you don't tend to see used much anymore, even though it's basically the Biomes O' Plenty of animals.
MAtmos, yet another old Technic/Tekkit mod, I haven't seen it used a lot anymore but I always liked the added atmosphere.