Yes, it appears to have been discontinued. Sorry.
Refer to the discussion at What happened to the Data Analysis Toolpak or Solver in Excel for Mac 2011? – answers.microsoft.com. Quote:
The Data Analysis Toolpak was removed in Office for Mac 2008. [emphasis mine] However,
the following is a free third-party tool [...]
[... goes on to mention the StatPlus software you already mentioned ...]
See also: I can't find the Analysis ToolPak – Microsoft Office Mac Troubleshooting. Quote:
Cause: Excel 2011 does not include the Analysis ToolPak.
Solution: Download StatPlus:mac LE for free from AnalystSoft, and then use StatPlus:mac LE with Excel 2011.
That last link used to include loads of comments from disgruntled Office for Mac users, but Microsoft seems to have removed comment functionality from that help center article. Surely you can find disgruntled comments re: the above elsewhere on the net.