Was the implementation of artificial gravity ever seriously considered for use on board the ISS and its derivatives? If not, why?
Yes, as far as the ISS is concerned, there were at least two such projects in the past that I know of:
- ISS centrifuge demonstration as part of the Nautilus-X (Non-Atmospheric Universal Transport Intended for Lengthy United States Exploration)
- Centrifuge Accommodations Module that was designed for NASA by JAXA
None of these seem to be going anywhere, for now, but you can say that they were seriously considered. The latter (CAM) was even partially built and is now sitting as a sad reminder to budgetary cuts and plan changes as administrations change at the Tsukuba Space Center's outdoor exhibit.
There were also some non-related to the station proposals, such as Zubrin's Mars (Semi-) Direct variants that suggest rotating a tethered formation using a spent upper stage as counterbalance mass, and Nautilus-X on its own as MMSEV (Multi-Mission Space Exploration Vehicle). There might be many more similar concepts under consideration that I can't think of now. Of course, there are always the "space wheel" stations that popup in various discussions, but I'm not aware of any being seriously considered (whatever that means, let's settle for some significant money being raised or spent on it) for the time being.