See MC-51953 for a full description.

The issue is NOT about FOV, NOT about bobbing, it is about DISTORTED rendering, as can be clearly seen in the youtube video:
https://youtu.be/yraaPpr3qGo

Especially notice the left and right parts of the screen; as the player is looking upward and downward, rendered parts are extremely distorted in a kind of sideways parabola.

Even looking at that video will give me extreme nausea.

It has been 4 years since I submitted the original issue, it was closed without a solution.
I still cannot play Minecraft without getting sick, just as lot of other people.

This was NOT happening when I bought the game; I could play it for hours on end!
Similarly, this does NOT happen to me when playing Skyrim or Oblivion; I can also play that for hours on end WITHOUT any problems.

Please look into this and come up with a solution.

UPDATE: 2021-07-10

Given that the original bug (MC-51953) was submitted more than 7 years ago, and was set to resolved, without actually solving it, and this bug has been multiple times been set to resolved and reopened again, this effect causing motion sickness for many people is apparently not important to Mojang/Microsoft. Sad.

Also it's amazing how many people keep repeating it's a FOV issue/pixel blurring issue/graphics card setting issue, while it's not about that. Even the YouTube video clearly showing the distortion effect near the left and right edges of the screen, saw a recent comment from somebody telling he saw nothing wrong with it. The issue is about distorted rendering which is most visible near the edges of the screen, and most visible for objects with straight lines that are close by.