When is the last time you lost to a split push / backdoor?

It's less common because the meta is more geared towards teamfighting so split pushing has fallen as a less favorable strategy. Still completely viable, in fact I was on a team that won due to it a couple times in the last few days. Takes a very specific champion for it though.

just yesterday

i am the splitpusher

Fast, heavy hitters with a quick escape can do it. Udyr, Trundle, Tristana, Twitch, Quinn (any ADC, really, but ones with attack speed boosts and speed buffs can do it better than others.), Darius, Wukong, Tryndamere and Vi, to name a few.