Well Scite excludes a lot by default, but markdown is defined for it. From `embedded.proterties`:
``` imports.exclude=abaqus asl asn1 au3 ave avs baan blitzbasic bullant \ cmake cobol coffeescript csound ecl eiffel erlang escript flagship forth freebasic \ gap haskell hex inno kix latex lot lout \ markdown maxima metapost mmixal modula3 nim nimrod nncrontab nsis \ opal oscript pov powerpro powershell ps purebasic r rebol rust \ scriptol smalltalk sorcins spice specman \ tacl tal txt2tags verilog vhdl visualprolog ```
Guess it probably excludes the ones the Scite maintainer doesn't know or use, little known things like `rust` for example :)
As the feature request you posted points out, many lexers are contributed, so updates also need to be contributed. The Scintilla maintainer can't know and use all languages. Thats why I subtly suggested with patches :)