Q: Matte feature for Photos in OS X El Capitan

Did you check with BorderFX developer if they have a Photos-compatible version?

So far Borders FX has not produced an updated version - a few others have created extensions to do borders and mattes  --  FX Photo Studio and Super Borders are a couple to look at  - Photo Editing Extensions for Photos for Mac

and you can check with the developers of Borders FX to see what their plans are

LN

I don't profess to be an expert but reading through these forums I think you can still use iPhoto in El Capitan.  If your photo has not been amended by Photos you should be able to access it from iPhoto and use your existing software within it.  iPhoto should still reside under Applications.  Let me know if this works as I am just getting to grip with the same problems myself!

You can use iPhoto is you are up to date - it required version 9.6.1

You can NOT access the same photos using both iPhoto and Photos - you would have to export it from Photos and import into iPhoto and do the matte

The solution is  to use extensions as posted above

-  FX Photo Studio and Super Borders are a couple to look at  - Photo Editing Extensions for Photos for Mac

LN