It should be fair to notice that the 15th anniversary edition is Windows only. There is a download link for the anniversary edition in the Mac OS X tab, but it's still an exe file which can't be run on a Mac without special software.

I only see the 20th Anniversary edition but no link to the 15th, and I know I could download it in the past

Indeed. My account does not have that link too.

I don't know why, but I too can confirm that the 15th Anniversary Edition has been removed.