When asked what advise he would give to players of modest strength, Anatoly Karpov replied: (paraphrase) Don't forget the old games. Many lines of play that have fallen into disuse can be dangerous weapons because players have not studied there half forgotten secrets.😏
Most GM's know all of the old lines and new lines played in each opening. I have heard the only way to understand the current theory is to learn the old theory. For example Nc6 instead of d6 use to be played in the sicilian defense, but after the bishop pin on the knight lead to d6 before Nc6 move order.
Modern GM's know these games. They are taking their old ideas
and developing them into modern weapons.
Also a lot of the old great combinations are never seen these days
because both players know the positions that breed them.
Two GM's can sit down and play out theory for 20 moves.
This is because the past Masters blazed the path they are on.
These two GM's will be aware what happens and what to do if
one deviates. This comes from studying and knowing the games
of the great (and not so great) players.
You very very rarely see a copycat game at GM level. A game played
move for move from a game played in the, say, 1920's.
Top players know what to do and what to avoid because that have
looked at these older games.
However. It is not uncommon, infact it happens everyday, for weaker
players to fall for the same opening traps players were falling into
in the 1800's.