If you want to wipe out 80% of the world's population with a disease that has an 80% fatality rate, then every person on the planet (or close enough to round up to 100%) has to catch it within the designated time period.
Normally I'd agree with others who have written about the horrific flu epidemic of 1918. If you made that flu even worse than it already was, the death rate would have been far higher. And it already was quite high.
But if you're going to infect every person on the planet, you need to come forward in time where pretty much everyone travels "to town" regularly, most people visit medium to large cities at least a couple times a year, travel over a few hundred miles is also common, and international travel happens enough that everyone will meet a few people that have traveled internationally lately.
Perhaps the 1970's.
Anti-viral treatments weren't that great (they're not all that great now but they exist) but travel was common and most people owned cars. Flu vaccines were around in the 1930's but, even now, they're ineffective against any strain not directly included. If the companies making the shots are blindsided by this upcoming strain (which is likely), the shots won't help anyone.
What will get you is the effective and near instant communication of the 1970's. TV and radio. A lot of people will be saved from catching it by being warned. But this would be true in earlier times as well. In 1918 there were telegraphs and daily newspapers. A delay of one day (or maybe only 12 hours) in announcements won't make that much of a difference.
But if you (frame challenge) change the incubation period, you can achieve your goal. Make it 6 months, or even a year or two.
Look at something like AIDS. The very long incubation period (and initial lack of a diagnostic test during the incubation period) is what allowed it to spread as much as it has, even though HIV is really not very contagious (you need direct blood or sexual contact and even then transmission rates are low (the average risk of contracting HIV through sharing a needle one time with an HIV-positive drug user is 0.67%), plus it doesn't survive outside a host more than a few hours in almost all cases).
The 1970's works well here because you're still in the "free love" stage in the US and some other countries, AIDS hasn't hit yet so those skill sets aren't around, it's been a long time since the Spanish Flu, and epidemics like polio and smallpox are mostly gone, giving a false sense of security.
Replace HIV with your nasty-flu, with its high contagion rate, add in a long life outside of a host, and a very long incubation period, and you might just get your pandemic.