So when I play Minecraft, I usually switch between different versions because I play on a large variety of different servers. For example, using 1.12.2, 1.8.9, 1.14.4, and 1.15.2. I will usually go onto 1.8.9 and start playing PVP (before the combat update). Then, I want to play my survival world that I have saved in 1.14.4. The keybinds are still the same, but when I switched back, all my keybinds were set back to the default. After some testing, I found out that version 1.16 through 1.13 kept keybinds that were stored into your settings alone, while 1.12 and below kept the keybinds as well, but if you were to go into a newer version and go back to 1.12 or under, the keys would be restored. I can't really show screenshots of it but you can try it out yourself to see.

Summary: Keybinds saved in 1.12 or under, and then playing 1.13 or above, would then resulting in the keys being set back to their default settings when returning to 1.12 or under.