HDDs are still in use. We use them plenty at work because SSDs are nice and all but when you need many petabytes of data and need multiple tiers of data, HDDs are still far cheaper.
As for mainframes. Yeah, not really. They're still huge with various companies. I've watched companies spend millions upon millions trying to do one thing; replace their mainframe. The main problem with mainframes is lack of people to support them. I'm not sure I know anyone familiar with mainframes in the US that's under the age 50. I've tinkered with them in a mainframe emulator (Hercules) running MVS 3.8j as well as having had access to them at work but IBM mainframes, at least, are a different world of computing.
For my list.
SNA - I was so glad when some companies we work with killed their SNA requirement. I had to support SNA services running on AIX. Supported it on Solaris at a previous job. What a pain.
VMS
AOS (Data General's competitor to VMS)
VSOS (Wang's competitor to VMS)
Dynix/ptx (Sequent's Unix, bought by IBM)
DEC Alpha - remember how those were going to be the future?
SGI Irix
Cray Unicos
DR DOS
Amiga - I kid, sort of
Atari ST
DB25
ATDT! Hey, if we're doing AT commands, we need that one.
