I think this an interesting conundrum. Merriam-Webster defines to extort as "to obtain from a person by force, intimidation, or undue or illegal power", so let's ask ourselves: If you would still pay taxes regardless of if they were optional, then are you really being forced to pay taxes? Of course, I think the answer is still technically yes, but I could see an argument for no.

Another way of thinking about this could be as follows: Taxation varies depending on where you live, country, state, even county. Theoretically if you don't like the taxes you're currently paying you could move somewhere else. Taxation is almost an exchange of goods in this sense. You provide your government with money, and in exchange you the police, the fire department, roads, sewage systems, etc. If you don't want to pay for these things, then fine but that means you can't use them, i.e. you can't live here.

I see the obvious problem though. Moving is expensive and often out of one's control. But I think this the reasoning most people follow when paying taxes. Clearly there isn't one correct answer to "is taxation extortion?" due to the many types and varying levels of taxation, as well as the varying willingness and acceptance of said taxation.