Is "than years gone by" grammatical?

Today, it is fairly common for people to take in more calcium than years gone by

I found this sentence but I thought it should be …than in years gone by

What do you think? Is it still grammatical?

Both you and Codeswitcher are correct that:

Today, it is fairly common for people to take in more calcium than years gone by

is not grammatical, and needs an in:

Today, it is fairly common for people to take in more calcium than in years gone by

There's an additional grammatical error in this sentence. The use of fairly is not correct, because it expresses degree without any comparison, but the sentence is describing the relative levels of calcium intake now versus some time in the past. Fairly should probably be replaced with [relatively] more.

In years gone by is a rather poetic idiom meaning the way things were some time ago. It vaguely refers to an era many years (decades or even centuries) ago, and not to the recent past or any specific period of time. It's used to describe the way things used to be, and is quite similar to in the old days in meaning.

That in is important because it's what adds the sense of during to the phrase. Without it, the target of the comparison becomes the number of years gone by, which makes no sense, though it is grammatical.

The only way I can think of to create a semantically correct use of than years gone by would be for the speaker to be directly comparing some other quantity against their age. For example, an old man saying:

I have more aches than years gone by.

This poetically expresses how vast the number of pains afflicting him is. Note that this is a very peculiar turn of phrase, and outside of grammatical errors and this contrived example I've never encountered it.