I installed El Capitan 10.11.5 on my mSata SSD drive, even migrated all data from the old 10.11.4 hard drive and my Brother MFC-665CW no longer works due to missing driver. Now, the original 10.11.1 had the driver, if I upgraded to 10.11.2 then 10.11.5, the driver is still there and I can still use the printer. Brother websites confirms no support on these legacy MFC-665CW on El Capitan 10.11.x.
My MFC-665CW is still working well with wireless connections, but I think it’s time to move on to the newer version that supports AirPrint for our iPad and iPhones. The lowest version of Brother I can buy now is MFCJ460DW on Amazon.