What was the most outdated console while it was still being produced?

First off "outdated" does not mean "not fun", we at DP know that even games that are 40+ years old can still be fun. All you need to enjoy a video game is a "ball", a "net", and two "paddles".

The obvious answer may be the Sega Master System because it is 33 years old, consequentially having 8-bit era technical specs, and is still being produced in Brazil. But that's just one country that makes up a fairly small percentage of the worldwide video game market.

For the USA, I would say the Atari 2600 is a strong contender. The timeline linked has its production ended in 1988, but there were games coming out for the system (as in, retail games, not homebrew) up to 1990. Super Mario Bros. 3 is the game that epitomizes 1990 in my opinion, being by far the best-selling game that year and a good representative of the state of "typical" video game graphics and sound then - an NES game, but a "high-end" one on graphics, sound, and advanced gameplay where the programmers had harnessed almost all of the capabilities of the system. The 2600's most advanced games came out between the immediate pre-Crash years (c. 1983) and 1990, with such games as Pitfall II and Solaris being the high watermark of what was available for the system during its retail life, but looked massively outdated compared to Super Mario Bros. 3, or even the first-generation "black box" NES titles. The most advanced 2600 games couldn't even match the ColecoVision technically. There was a HUGE jump in the look and feel of Atari 2600 games to the late 8-bit/early 16-bit games coming out around 1990. The 2600 was a great system between 1977 and 1984, though it was showing its age after 1982. I often wonder if Atari should have even kept producing the 2600 after the 7800 came out, because the 7800 was backward compatible.

The Sony PlayStation is another one, I witnessed this one personally. Sony ended production on the PlayStation on March 23, 2006. By this point the Xbox 360 was out, and I was playing it. The 360's graphics looked nearly photo-realistic (at the time) while the PlayStation's are much simpler. Even the handheld consoles of the time, the Nintendo DS and PSP, out-powered the PlayStation significantly. Even some of the PS2/Xbox/GCN games coming out by 2005-2006 were a major leap from what those consoles were doing circa 2001.

The NES comes close, but it was discontinued BEFORE the PlayStation came out. As such, the 16-bit era was still "mainstream". The gap between NES and 3DO is probably just as big as Atari 2600 to NES, but the 3DO was only a small portion of the market.

The Neo Geo is another one. Sorry Neo Geo fans, I think it's a great system, but games came out for it in 2004, by which point the system itself was "retro". SNK and other developers milked 101% of the power out of that system with its later games, but the fact remains that a year off from the Xbox 360's release, the games looked nothing like what was coming out on PS2, Xbox, and GameCube. Of course, it wasn't competing head-on with those systems by that point, but releasing games for a console that had been out of production for 7 years to a dedicated fanbase who knew what they were getting into and enjoyed it.