What happened to electrolytic capacitors in the 21st century?
There was a period of time where lots of capacitors were made with a dodgy electrolyte, especially by some large Taiwanese manufacturers.  The capacitors looked OK in a wide variety of tests when new, but they didn't age well. Because it took a few years for the capacitors to fail, and the high failure rate to become known, an awful lot of them had been produced and built into things before people realised there was a problem.  It then took a few more years to for the things to leave circulation.

Exactly why these manufacturers had electrolyte problems is not completely clear.  They were using new, water based electrolytes which had been developed in Japan and worked very well. Presumably the cheaper manufacturers had missed something or cut some corners while reproducing (or ripping off) the Japanese research.

The type of capacitor affected was cheap, large capacitance, low ESR capacitors. These are the kind of thing that appears in huge numbers of consumer devices, so the problem became known in the wider community. Plus, the failure mode of these capacitors was rupture and venting, so it was easy for even people unfamiliar with electronics to see which component was at fault when their motherboard stopped working.

Wikipedia has an article about it: Capacitor Plague