I think we take it for granted how important this accolade is :good "many pacquaio is the only boxer who is credited with lineal championships in four different weight classes" Muhammad ali is the only heavyweight champion to hold the lineal championship three times, beating Sonny Liston (1964), George Foreman in a comeback from a three-and-a-half year forced retirement (1974), and Leon Spinks in an upset by unanimous decision (19788).
Sometimes it means a lot, sometimes it looks better on paper than it really is. George Foreman became the lineal champion in 1994, and stayed that way by facing fringe contenders until being robbed against Shannon Briggs. Michael Spinks was the linear HW champ when he fought Mike Tyson, but Tyson was the one who was facing better opposition. Tyson was the active fighter cleaning out the division. Ali holding it 3 times is overrated, since it involved him losing to a guy with 7 pro fights. Mind you he was a shot fighter about a dozen years past his prime, but for all the great achievements Ali has, holding it that 3rd time doesn't really rank high up there.
Manny Pacquiao did. However, your post said that Pacquiao is the only fighter to have done it. That's not true anymore. Two fighters have done it.
And done it without beating the main men in both those divisionsatsch Lineage means **** in this day and age.