
.
Some did, some didn't.
Most of the old time strength work was wood chops / sledge hammer work, weighted drags/carries, and basic bodyweight stuff. You can sometimes find video of 1970's or early boxers lifting weights, but it's very moderate volume and intensity. It's even like that today for the most part.
The big push toward heavier strength training happened late 80's, very early 90's with Evander Holyfield, Foreman's comeback (that''s when he claims he did the bulk of his lifting), etc.....