https://dlang.org/spec/ddoc.html does not mention DDOC_MEMBER_ANCHOR, but https://github.com/dlang/dmd/blob/master/res/default_ddoc_theme.ddoc has it, and the the dlang.org repo's html.ddoc has it (though it purposefully sets it to nothing).
Only DDOC_MEMBER_HEADER seems to exist now - issue 18060.
Maybe you meant DDOC_HEADER_ANCHOR?