What happened to the standard MacBook series?

If you are referring to "plain" MacBooks, Apple released nearly a dozen models of "plain" MacBooks between May 2006 and May 2010. They discontinued the last of those by July 2011. These were laptops that weighed about 5 pounds (more or less) and that had both 2.5" mechanical hard drives and 5.25" optical drives. All of these "plain" MacBooks were released before Thunderbolt 1 came out, so none had Thunderbolt of any type. Or USB 3 for that matter. (Most did have FireWire 400 in addition to USB 2.)

Apple released 12" Retina MacBooks in April 2015, April 2016, and June 2017. These were two-pound laptops that used low-end Intel mobile CPUs, had soldered-in SSDs, and that did not have built-in optical drives. These Retina MacBooks had two ports: a USB-C (USB, DisplayPort, Charging) port that did not support Thunderbolt, and a 3.5" headphone jack. They also had the infamous "butterfly key-switch" keyboard. Apple discontinued the last one of these models In July 2017.

The MacBook Neo is, in some ways, very much like the 12" Retina MacBooks. Some improvements:

• The A18 Pro is much faster than any of the Intel processors in the 12" Retina MacBook Pros ever were.
• The port selection is still very limited, but that second USB-C port (which is for USB 2 and charging only) may simplify port juggling, especially when one is "on the run".
• The keyboard presumably uses "scissor-switch" mechanisms instead of the unreliable "butterfly-switch" ones.
• It is much less expensive. A MacBook (Retina, 12-inch, Early 2015) with 8 GB of RAM and a 256 GB SSD listed for $1299 USD, which is about $1783 in 2026 USD when adjusted for inflation. A 8/256 GB MacBook Neo would set you back $599 USD. So in this particular base price comparison, the Neo is about 1/2 the price in absolute terms and 1/3 the price in inflation-adjusted terms.
• It can run modern versions of macOS and modern Mac applications.

Not everything is better.

• Like the 2015 and 2016 MacBooks, the MacBook Neo is only available with 8 GB of RAM. The 2017 MacBook could be ordered with 16 GB of RAM.
• The MacBook Neo weighs about 2.7 pounds – the same as the 13" M-series MacBook Airs. So it isn't quite as portable as the 12" Retina MacBooks, which weighed about 2 pounds.
• The MacBook Neo does not have a backlit keyboard.

The standard MacBook was retired in 2019. The newly launched MacBook Neo should more than adequately replace the old MacBook.