Classic Mode is the first mode of Google Snake that was created. Apples will appear on screen one at a time, with the new apple spawning in a non-occupied square when an apple is consumed. To beat this mode, you must eat 252 apples. Your first game will automatically be on classic mode.
To keep things short, the optimal strategy is the shortest path to any apple. If you have fast enough reaction you can turn in time, but if you don't, just go for the middle of the board and turn to the apple as soon you are able to react. In fast mode reacting in time is even harder, so you should practice and keep trying.
This is true for any non-wall mode up to 50 apples.
As for Wall, you should actually prioritize surviving and path finding, as the shortest path to an apple is very likely to be an avoidable death trap. I'd go with this from wall 50, and for any mode 100 or above category.
If you run into a death trap in wall mode that is unavoidable, please join the discord and post it, as it would be considered a bug and should be fixed. An example of one would be where you have to die to get the apple, no matter what, this shouldn't happen.
World Record progression
All dates in UTC
12th January 2020: 28.333 seconds by Yarmiplay
24th October 2018: 1 minute 33.00 seconds by Franzcorr
1st July 2019: 1 minute 31.20 seconds by Franzcorr
5th July 2019: 1 minute 28.00 seconds by Yarmiplay
30th October 2019: 1 minute 26.55 seconds by aziangooglesnake
2nd December 2019: 1 minute 24.22 seconds by onetabloid
11th Decmeber 2019: 1 minute 22.00 seconds by Yarmiplay
13th December 2019: 1 minute 20.08 seconds by aziangooglesnake
15th December 2019: 1 minute 19.40 seconds by Yarmiplay
24th December 2019: 1 minute 13.367 seconds by aziangooglesnake
4th June 2020: 1 minute 12.77 seconds by Yarmiplay
26th May 2019: 5 minutes 04.06 seconds by Conquerr
7th December 2019: 4 minutes 50.00 seconds by Yarmiplay
8th December 2019: 4 minutes 41.00 seconds by Yarmiplay
9th December 2019: 4 minutes 24.00 seconds by Yarmiplay
12th December 2019: 4 minutes 02.00 seconds by Yarmiplay
17th December 2019: 3 minutes 45.20 seconds by Yarmiplay
24th January 2020: 3 minutes 30.61 seconds by Yarmiplay
30th April 2020: 3 minutes 25.99 seconds by Yarmiplay
21st February 2020: 19 minutes 58.17 seconds by Yarmiplay
21st February 2020: 13 minutes 37.64 seconds by Yarmiplay