Well, I'm not sure about a few years exactly, but I've always thought of this question in terms of wildlife. The first thing that comes to mind are species lost from environmental destruction.
One example: whale song is changing. One theory is that they can no longer communicate as they did because marine traffic overwhelms their language.
Bernie Krause ...
Some props will also become scarce over time; keeping typewriters functioning is getting challenging. I use my library of thick phone-book hits for all sorts of stuff, which also causes them to get duct-taped together and often nearly destroyed. What will I use when everyone stops publishing thick pulp softcovers? Resort to punching interns in the stomach?
Phone work ups. Now everyone uses a smart phone in films. flip phones, antennas, function beeps, and key clicks I can see being endangered. I feel like I haven't heard a wilhelm scream in ahwile either.
This is probably a sound already in someone's library, but I remember my parent's old 50's era (or prior to that?) electric coffee percolator. When it is left to perc for 30 mins. to an hour (adding additional water here and there) it has quite a vocabulary.
Fun question! True - you can find many of these sounds in sfx libraries. We have recordings of NASA space shuttle launches at Pro Sound Effects, and recently have been making 'sound a day' audio posts about sounds of endangered species.
To my great dismay: space shuttle launches. Supposedly one of the (if not THE) loudest man-made sounds. I know the Transformers 3 sound team was able to record the last launch—quite a priceless recording if you ask me. Of course NASA and private companies will be launching other forms of rockets for years. I doubt many would be able to tell the difference.