I have a 4" thick, roughly 6' long maple slab that I am making into a headboard. I am about halfway through the sanding process and am thinking about the finish. I don't want stain, because I want the natural look of the wood, and I don't want anything shiny like polyurethane. I am considering tung oil, but I know it will have to be re-applied yearly. I wouldn't mind a flat or matte finish. I'm looking for suggestions.
This is the type of thing that should be asked "what finish could I use...." rather than should. There are essentially no areas of finishing where there's a should use, almost every case comes down to what the builder chooses to use that fulfils the requirements as they see them.
The same on-paper requirements are guaranteed to lead to different woodworkers going with different finishes, as the many varied finishes used on coffee table projects attest.
As a headboard doesn't need the kind of durable coating that something like a coffee table might need you could finish this with just about anything, including nothing at all believe it or not.
I am considering tung oil,
An oil finish could work really well for something like this. First time I ever used an oil to finish a piece of furniture I didn't have in mind any particular requirement for durability, but now a few years down the line I'm constantly impressed with how it has held up to wear-and-tear. This is because it was applied to a hardwood and maple is of a similar hardness so you could expect similar performance.
However, I wouldn't recommend you go with tung oil unless you already have some. The much-sung benefits of tung are nearly entirely about its outdoor performance. Boiled linseed oil, BLO for short, has a long and glowing history in Western woodworking, is more than adequate for indoor use and is commonly cheaper and more easily available. It's also reputed to be easier to apply to a good finish (source: Bob Flexner).
A additional benefit of boiled linseed oil is you know for sure what you're getting — just linseed oil, treated to make it dry faster. On the other hand many products that have the words tung oil in the name are a bit of a purchasing minefield as they may only have some tung oil inside, and a few contain no tung at all!
I know it will have to be re-applied yearly.
That's one tradition, but it's actually not a must-do.
The piece I referred to above has not had another coat applied to it since its initial six or seven a few years ago. And as far as I can tell it looks exactly the same, including on high-wear edges.
I don't want anything shiny like polyurethane.
An oil finish can turn out quite glossy, particularly on dense hardwoods, so oiling is not a direct route to a matt finish.
While I recommend BLO above it does have a negative in this case in that it is usually darker in colour than tung oil, and will impart more of that characteristic yellow tone to lighter woods like maple which you may not care for.
What I would usually recommend in its place is varnish. Polyurethane, as with any varnish, doesn't have to be shiny.
And by thinning it to turn it into wiping varnish it can be applied in much the same way as oils are — wipe on, wipe off any excess. Even using gloss varnish when applied this way you won't get a particularly gloss finish unless you build up many coats, e.g. more than four. If you do end up with a surface a bit shinier than you'd like you can just go over it gently with fine Scotch-Brite or steel wool to 'knock back' the shine.