I never have seen a traffic/keyword domain (that I could believe was real) that I could afford. Maybe that colors my belief that temporarly depressed market segments will in the short/medium run outperform keyword / traffic domains. An example would be LLLL.coms, until they get into balance with the overall market. It is hard to buy keyword / traffic domains below market - they are too visible.
As for the OP --- recession-proof? I doubt much of anything is, but the internet has been growing at a rapid rate, and there is no reason for that to stall. Last Christmas was a down year for retailers, but online sales were up 20%. So I do not see us going backwards unless things get really bad in the outside world. But outside world factors will force some domainers to sell cheaper than they would want to, so there will be some aftermarket bargains.