"What, it is 2017 and you design a deck in 4:3 format?", I got these questions a few times. Here are the pros and cons of both formats.
A 16:9 or widescreen aspect ratio will give you a nice image on an LCD conference room monitor or desktop/laptop screen with the black bars on the left and right
A 4:3 aspect ratio will look better on projectors, which are still used in many larger presentation rooms. Also: 4:3 looks better when decks are printed, a habit that is still very common in the financial services industry where people like to take notes, look in detail at data tables, (and probably want to take an opportunity to quickly flick ahead if the presenter is slow/boring).
And personally, I like the design freedom of a more even design canvas (4:3) better than the wide screen version, which forces me to make horizontally stretched slide designs. (A cheat: put the headline across a number of lines to the left of the slide and use the imaginary 4:3 canvas to the right of it for your slide content.
So, here you have it. I don't think 4:3 is old fashioned for presentations (it is for movies), it just depends on the most likely presentation context you expect.
In my presentation app SlideMagic, I used a 4:3 canvas, but use the extra horizontal space of a 16:9 screen to add your "explanation boxes" that you can slide in and out. When set to "out", the presentation becomes 16:9 with a more detailed description of the slide in case you send the document ahead of a meeting and the recipient will open/read it without you being there to explain it.
Image via WikiPedia