You would say 800 years history, plural "s" is required, feel pretty strong that is correct.
Just like you don't go 60 minute ago, it's minutes, years for more than one year.
Basic English, OK, except that you didn't put the apostrophe in
800 years' history.
If someone said to you, "The cathedral has been open for 5 year." Would you find that wrong or right?
If you would find it wrong, how is your example any different?
That is definitely wrong in standard English, though acceptable in some dialect versions.
I'm not sure which example you are talking about - you haven't made that clear.
Maybe you are referring to
one-year history?
In that expression,
one and
year are hyphenated to form an adjective: so we may have a
one-year history, a ten-year history, a seventy-year history.
In
one year's history,
year is a noun and takes the singular possessive form in this context. Because year is a noun, for larger numbers it must become the plural possessive form.
So we have
two years' history, ten years' history, and so on.
So let's see, "The cathedral has 2 years' history of choral excellence"?
Correct.
"The cathedral has 1 years history of choral excellence"?
Incorrect, the apostrophe is essential:
"The cathedral has one year's history of choral excellence."
A year is history, just like a decade, a century or eight.
I don't agree.
In this context, a year is a measure of the duration of the history, as are decade, century and eight years.
History can be measured in these terms, just like experience.
You would not, surely, say that 'A year is experience ... ...'?