My understanding is that Lightroom 6 (six) can still be purchased and activated as standalone perpetual license.
My question is, can Lightroom 5 (five) still be activated?
Depends on what you mean by activated.
True Software Activation was first added to Lightroom version 6. There is no activation (as Adobe uses the term) in earlier versions.
If you mean can version 5 be installed and the serial number be entered so that the software is licensed for use on that computer then the answer is yes. That is true for all versions back to version 1.
Thank you for the clear and helpful reply (sorry for my late comment)