This is the normal version used throughout loader.c which was added for the steamclient swap:

https://github.com/ValveSoftware/wine/blob/2409bd1f74be116172688a25df725290637c255a/dlls/ntdll/loader.c#L1618

static WCHAR *strstriW( const WCHAR *str, const WCHAR *sub )
{
    while (*str)
    {
        const WCHAR *p1 = str, *p2 = sub;
        while (*p1 && *p2 && tolowerW(*p1) == tolowerW(*p2)) { p1++; p2++; }
        if (!*p2) return (WCHAR *)str;
        str++;
    }
    return NULL;
}

And this second one was added specifically for the AoE3 game check:

https://github.com/ValveSoftware/wine/blob/2409bd1f74be116172688a25df725290637c255a/dlls/ntdll/loader.c#L206

static WCHAR *strcasestrW( const WCHAR *str, const WCHAR *sub )
{
    while (*str)
    {
        const WCHAR *p1 = str, *p2 = sub;
        while (*p1 && *p2 && tolowerW(*p1) == tolowerW(*p2)) { p1++; p2++; }
        if (!*p2) return (WCHAR *)str;
        str++;
    }
    return NULL;
}

It's the exact same function with a different name.

Used specifically for:

https://github.com/ValveSoftware/wine/blob/2409bd1f74be116172688a25df725290637c255a/dlls/ntdll/loader.c#L3205

            strcasestrW( libname, mfc42W ))
            
            

So you can remove the duplicate function and change strcasestrW on that line to strstriW

Also for future proofing, you can use RtlDowncaseUnicodeChar in the function instead of tolowerW:

static WCHAR *strstriW( const WCHAR *str, const WCHAR *sub )
{
    while (*str)
    {
        const WCHAR *p1 = str, *p2 = sub;
        while (*p1 && *p2 && RtlDowncaseUnicodeChar(*p1) == RtlDowncaseUnicodeChar(*p2)) { p1++; p2++; }
        if (!*p2) return (WCHAR *)str;
        str++;
    }
    return NULL;
}