Looks like you can do this from Settings ... Shortcut Mapper ... Scintilla Commands.
The current behavior for Ctrl-L appears at the bottom; you should be able to set it to Ctrl-D, but be mindful that you'll have to unset the current mapping for Ctrl-D, which appears further up in the Scintilla Commands (unless it unmaps it automatically).
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Is it possible to change the shortcut for deleting the entire line to Ctrl-D instead of Ctrl-L?
Looks like you can do this from Settings ... Shortcut Mapper ... Scintilla Commands.
The current behavior for Ctrl-L appears at the bottom; you should be able to set it to Ctrl-D, but be mindful that you'll have to unset the current mapping for Ctrl-D, which appears further up in the Scintilla Commands (unless it unmaps it automatically).