I've run Mavericks, Sierra, and now High Sierra on unsupported Macs following instructions on the MacRumors forums. The latest on a MacBook (2008 Black) was Mavericks, and it ran just fine. I had some issues getting Sierra working on it, but I was trying to not have to do a clean install. In theory, Sierra should run on that hardware, but I've not tried recently.
After installing Sierra on a 2009 Mini, and High Sierra on a 2009 iMac, I think the best path is to:
- Backup using CCC (SuperDuper for some reason will not work on an unsupported High Sierra installation for me) to an external hard drive. This will be your clone for a quick restore.
- Backup using TimeMachine. This will be for your migration.
- Nuke and Pave, using your (Sierra | High Sierra) installer you made.
- Restore from your TimeMachine backup during the backup process.
If things go wrong, you've got your CCC backup that you can boot from over USB to assist with repairs, or to restore.
NB: #1 and #2 are not optional, and they are not "either/or". Do both.
Once you've done the install, there's nothing at all unusual about running the Mac, save for OS updates on High Sierra, which you have to get the combo updater to effectively do. Sierra updates just ran fine straight out of the Mac App Store. Do remember that SIP is disabled, so you might consider an anti-malware package in addition to XProtect, if only to run it periodically. (Since we're dealing with older Macs, every cycle of CPU is precious!)
Written on my 2009 iMac running High Sierra,