SIMBL lives on mostly thanks to w0lfschild, see: https://github.com/w0lfschild/MacForgeFramework. This repo hasn't been updated since 2018; I'm not entirely clear whether the most recent versions of MacForge is still based on SIMBL.

The SIMBL on my website uses an unmodified SIMBLAgent binary taken from an older version of w0lfchild's mySIMBL (https://github.com/w0lfschild/mySIMBL). However, I recompiled his SIMBL.osax with 32-bit support added back in, since I use a lot of 32-bit apps: https://github.com/Wowfunhappy/MacForgeFramework

Quick note that I don't really understand this code, I can just tell you that it works, at least on Mavericks.