Everyone moved on to... somewhere.
The majority to
SMBX2 since it is built around the original Engine, thus offering good backward-compatibility while adding tons of new content. However, it suffers from a lack of accessibility, partially due to it's relatively high hardware-requirements.
SMBX-38A doesn't require powerful hardware, which is one of the reasons why many people moved there. It's a Remake of SMBX that was built from the ground up. It also adds new content, but has bad backward-compatibility. Episodes made on 1.3 are not guaranteed to work on 38A.
In 2020, the source-code of SMBX 1.3 got opened, and then ported to C++ under the name of
TheXTech. It offers the best accessibility, being able to run on a large number of Platforms (including Android and even some consoles) while having lower hardware requirements than either 1.3 or 38A.
All of the three add various quality-of-life features. One example shared by them all is the
SDL Mixer X library, which allows to use a wide variety of music formats, including Chiptune Soundtracks from old Nintendo/Sega systems. This allows to replace MP3s with SPC files that offer pure quality at a significantly smaller filesize.
At this point, the only reason to keep using 1.3 are research purposes to improve the other versions.