FG% has declined because people take more three-pointers today.
Also, old school teams turned the ball over more and were worse free throw shooters.
Even if they had three point system in the past the FG percentage wouldn't really change, plus wasn't dunking banned ? even if teams could dunk the FG% wouldn't change, unless defense softeners up like today's NBA, some say the no hand checking physical defense is the reason teams are shooting at a higher percentage in today's NBA, if the game was still physical and hand checking was allowed the FG% would be basically the same.
It's basically equal throughout the history of the NBA the stats don't lie.
Free throws have nothing to do with FG% percentage, that's FT% percentage, that would just be added to the points percentage scored in a game the points scored would be higher instead of 100 points average it would be like today's 117 points average as an example.
So it's basically still the same except the defensive rules allow more scoring, unlike in the 80s teams were allowed more physical play, in the 80s teams were allowed to be super physical but that's even exaggerated, I haven't seen film of old school super physical basketball play.
You will notice the FG% has increased over the years because of the defensive rule changes, otherwise more physical defensive play allowed the team FG% percentage would go down from 117 points today to probably 101 FG% average like in 2001 with the Lakers, since physical play hand checking isn't allowed, players can dribble and shot at a higher FG% percentage, teams naturally score more points a game.
Certain teams just don't play solid defense but that's always been throughout the history of basketball, like you said players are getting better at the three and taking more three point shots, but if the past players had the three point shot, the stats wouldn't really change anyway, it would still be balanced throughout the history of the NBA.
It's better to look at certain styles of boxing going against other boxers who had trouble against a certain style or types of boxers and the fighters limitations his weaknesses strengths chin foot speed size.
I don't believe Ali beats Mike Tyson, Ali is getting stopped late because of the style matchup and Ali having difficulty against that type of boxer with that style body type physical built, but Ali would out box Fury because he's just better than Fury at what Fury is trying to do, Ali has better footwork has superior hand speed and has power himself, I wouldn't be surprised if Ali stopped him late because unlike Wilder, Ali is the master class boxer.
Ali would do something Wilder couldn't to Fury outbox him, that's my opinion but you're opinion might be different that's why we discuss these things but we don't have to agree, I could totally be wrong and change my mind, and that's okay just like I changed my mind about Ali beating Mike, now I don't believe Ali could beat Mike, now what if Ali according to reports was starting to get sick when facing Smoking Joe ? because that's what his trainers said maybe Ali beats him easily if Ali wasn't starting physically have problems with his coordination his mental state, but we don't know if that had any effects on his boxing abilities, his mind.