So for compatibility for different versions of Notepad++ I suppose I will have to now copy the plugin to both locations:

You could also use the Win32 APIs GetFileVersionInfoSize, GetFileVersionInfo and VerQueryValue to query the version number from the file notepad++.exe in order to decide which target directory to choose.

I provide you some Delphi code that does the job. You should be able to port that to your preferred programming language.

Documentation of the VS_FIXEDFILEINFO struct : https://docs.microsoft.com/en-us/windows/win32/api/verrsrc/ns-verrsrc-tagvs_fixedfileinfo

// Enum to specify the type of numeric version info to query
TNumericFileVersionInfoTag = (
  nfvitFileVersion,
  nfvitProductVersion
);


class function TFileVersionInfo.GetNumericVersionInfo(const FileName: string; const InfoTag: TNumericFileVersionInfoTag; out VersionMajor, VersionMinor, Release, Build: integer): boolean;
var
  dwLen:    DWORD;
  dwHandle: DWORD;
  lpData:   pointer;
  dwBytes:  UINT;
  FileInfo: PVSFixedFileInfo;

begin
  Result       := false;

  VersionMajor := 0;
  VersionMinor := 0;
  Release      := 0;
  Build        := 0;

  dwLen := GetFileVersionInfoSize(PChar(FileName), dwHandle);
  if dwLen = 0 then exit;

  GetMem(lpData, dwLen);

  try
    if not GetFileVersionInfo(PChar(FileName), dwHandle, dwLen, lpData) then
      exit;

    if not VerQueryValue(lpData, '\', pointer(FileInfo), dwBytes) then
      exit;

    case InfoTag of
      nfvitProductVersion:
      begin
        VersionMajor := (FileInfo.dwProductVersionMS) shr 16;
        VersionMinor := (FileInfo.dwProductVersionMS) and $FFFF;
        Release      := (FileInfo.dwProductVersionLS) shr 16;
        Build        := (FileInfo.dwProductVersionLS) and $FFFF;
      end;

      nfvitFileVersion:
      begin
        VersionMajor := (FileInfo.dwFileVersionMS) shr 16;
        VersionMinor := (FileInfo.dwFileVersionMS) and $FFFF;
        Release      := (FileInfo.dwFileVersionLS) shr 16;
        Build        := (FileInfo.dwFileVersionLS) and $FFFF;
      end;
    end;

    Result := true;

  finally
    FreeMem(lpData);
  end;
end;

Now using v7.5.1 it is finding my plugin here:
C:\Program Files (x86)\Notepad++\plugins\AJCActiveBackupNppPlugin\AJCActiveBackupNppPlugin.dll

My product AJC Active Backup has the option to install the plugin and copies it to the the plugins folder. So for compatibility for different versions of Notepad++ I suppose I will have to now copy the plugin to both locations:
C:\Program Files (x86)\Notepad++\plugins\AJCActiveBackupNppPlugin.dll
C:\Program Files (x86)\Notepad++\plugins\AJCActiveBackupNppPlugin\AJCActiveBackupNppPlugin.dll

Notepad++ v7.6 (32-bit)

I do not recommend using that version. Versions 7.6 thru 7.6.2 were in a state of flux in regard to the plugin hierarchy, where they matched neither the old v7.5.9-and-earlier hiearchy or the v7.6.3-and-newer hierarchy. The plugin hierarchy stabilized in v7.6.3, and has stayed the same from then thru the current v7.7.1.

do you know ProcMon?
Set a filter to something like path contains AJCActiveBackupNppPlugin and see what is going on.

Debug info:
Notepad++ v7.6 (32-bit)
Build time : Nov 12 2018 - 23:51:42
Path : C:\Program Files (x86)\Notepad++\notepad++.exe
Admin mode : OFF
Local Conf mode : OFF
OS : Windows 10 (64-bit)
Plugins : DSpellCheck.dll mimeTools.dll NppConverter.dll NppExport.dll

Folder listings with my plugin in two places

Directory: C:\Program Files (x86)\Notepad++\plugins

Mode LastWriteTime Length Name

d----- 25/07/2019 15:47 AJCActiveBackupNppPlugin
d----- 25/07/2019 12:36 APIs
d----- 25/07/2019 12:42 Config
d----- 25/07/2019 12:36 disabled
d----- 26/07/2019 10:27 DSpellCheck
d----- 26/07/2019 10:27 mimeTools
d----- 26/07/2019 10:27 NppConverter
d----- 26/07/2019 10:27 NppExport
-a---- 02/05/2019 10:12 73728 AJCActiveBackupNppPlugin.dll

Directory: C:\Program Files (x86)\Notepad++\plugins\AJCActiveBackupNppPlugin

Mode LastWriteTime Length Name

-a---- 02/05/2019 10:12 73728 AJCActiveBackupNppPlugin.dll

It still does not find it.

Does it give an error message (often through a dialog box), or does it just silently ignore it? There are some plugins that worked with old versions of Notepad++ that don’t work under the new, due to changes in the Notepad++ codebase… but those would generally either load fine but give errors when you try to run one of their actions, or would give an error during the loading of the plugin.

Are you sure there aren’t any spaces in the DLL name? If there are, there need to be spaces in the path name as well.

Please supply the Debug Info (? Menu, Debug Info, Copy Debug info into clipboard) on the version that isn’t working. And, if possible, drop to a command prompt in your plugins directory, and give us a directory listing of the plugins directory and the subdirectory. paste into notation like

~~~
dir C:\Program Files (x86)\Notepad++\plugins
dir C:\Program Files (x86)\Notepad++\plugins\AJCActiveBackupNppPlugin
~~~

to wrap your output, so it renders like:

dir C:\Program Files (x86)\Notepad++\plugins
dir C:\Program Files (x86)\Notepad++\plugins\AJCActiveBackupNppPlugin

This will then make sure that the forum doesn’t mangle what you’re trying to show us.

So I tried it in:
C:\Program Files (x86)\Notepad++\plugins\AJCActiveBackupNppPlugin\AJCActiveBackupNppPlugin.dll

It still does not find it. If I reinstall Notepad++ v7.5.9 it works again.

Plugins that used to be directly in the plugins folder now have to be one level down, e.g.:

...\plugins\XXX.dll

needs to go in:

...\plugins\XXX\XXX.dll