On 18 Oct 2018, at 7:38, Lewis Overton wrote:

I use Textmate to prepare material for classes I teach. Most of what I do
is to write explations formatted with Github version Markdown. When OS 10
came out the formatting failed to produce the styled image. Has anyone else
had this problem?

The default markdown bundle uses Markdown.pl from John Gruber (not GFM) so presumably you installed a third party markdown converter and set TM_MARKDOWN, could it be that this one was lost/broken during the update to 10.14?

If that is not the case, please give some more info about what command you are using to produce the formatted Markdown and what error you are seeing.