Can you still do the free upgrade or have they put a stop to this?
Yes, you can still upgrade free from the existing installation of Win7/8 and you don't need to enter any license key. Skip this part. Actually entering a key would be a wrong (wasteful) idea. It activates automatically based on a digital certificate created during the upgrade.
Important: use a right disk image for the upgrade: Home versions only upgrade to Home, Pro/Ultimate to Pro. 32/64-bit version is up to your choice.
If you prefer to start with a clean installation first time (bypassing upgrade process), then in a case fo Win7 a key is needed, but rememer that only retail keys work that way (not OEM COA sticker keys attached to a computer case). For Windows 7 OEM computers (most cases) I do run a program gatherosstate.exe from Win10 ISO image which generate a digital upgrade certificate, then do a clean installation without entering any keys. Google for details.
When you upgrade from OEM Win8 computer (like HP,Dell, Lenovo), it is very simple. Do just a clean installation - once again - without entering any key. Win10 (v1709 or later) will pickup a right license burned in the BIOS.
Also remember that some OEM Win8/10 computers have downgradable rights burned in the BIOS. It means that if you are not happy wih Win10 (which came with computer), you can install older version, like Win7. To do so, you have to use OEM Win7 disk provided by the manufacterer. If you are clever, you can also install from a retail OS image available for download (once again without entering any keys), but after installation you have to follow a procedure how to insert OEM certificate.