The "Open Selected Version" context menu in Log/History uses the default application for that file type, but with several development environments installed on my machine, this is not what I want - I want to open in a text editor if I so desire.

Apparently there used to be (or maybe only on Mac version) an "Open With..." option (see https://jira.atlassian.com/browse/SRCTREE-1179)

Was this removed? Could it not easily be implemented? Or at least a "Save current version as" option?

There is no workaround using custom actions because there is no parameter that can access the "selected version" of a file rather than the "current version" (i.e. $FILE).

This was first reported in https://jira.atlassian.com/browse/SRCTREEWIN-2899 which was closed as it aged-out as an "unsupported version of SourceTree". A bit of a cop-out since the issue remains 8 years later with the current 3.4.12. The suggestion to re-open the ticket and update the affected version is not possible since the workflow does not allow this.