You may find some info here:
https://wiki.lazarus.freepascal.org/SynEditSynEdit exists for a long time. Afaik longer than Lazarus.
The version that is used in Lazarus (and shipped with Lazarus) today, was forked very long ago. It has been developed separately since, and has almost no code still in common with the original.
CodeTyphon is a Lazarus fork, and has a copy of the Lazarus version (Codetyphon is based on Lazarus svn trunk, so it contains some of the changes that will be in the next Lazarus version / Sometimes they make changes of there own, but I have not seen any big ones yet)
The original SynEdit is still on Sourceforge,
https://sourceforge.net/projects/synedit/ but has almost no activity.
Active forks on github exist.
There is probably work on a delphi version (based on the original)
There is also a Lazarus fork, based on a recent original version. I have no idea how much maintenance it receives.
https://wiki.lazarus.freepascal.org/SynEdit/portAtSynEdit is an editor component for Lazarus/LCL written from scratch, aiming to be similar to SynEdit.