Can I go back (previous versions)?

[akidd]

Simple question, can I go back to previous Mac OS versions?
Accept the this is unlikely on new late 2017 ordered models but older ones? On 10.16.6 on this 2015 MBP and there is software I'd like to run but cannot as it either wasn't updated or I don't have it.
Also can I split the HD on iMac so I keep the software versions I want and then have rest updated: HS will work on it (not sure I want HS but S might be fine) re recent security updates.
Or should just dig out old iMac which should work with those bits of software and use it machine by machine?

 

[Fishrrman]

For a 2015 MacBook Pro, you should be able to run El Capitan.
That's what my own 2015 MBPro 13" came with.

If you want to "switch boot" between two OS versions, I'd suggest you get a USB3 external SSD and use that as "the alternate boot drive". Fewer problems than running two versions on a single drive (but that can be done, too).

Generally, you can't boot a Mac with an OS version older than the one it first shipped with.

It -is- possible to run previous versions "under emulation" (using software like VMWare Fusion, Parallels, or Virtual Box). But that's quite a bit different than -booting- from them...