I think the 4000 series chips remain at the micron level of fabrication (\$3\:\mu\$ metal gate and not self-aligned) technology. As a percentage of operating FABs, there are less of them still operating at this fabrication level. But they exist. (I like to think of it as a "hand-me-down" kind of process, where older FABs still perfectly operational and more cheaply supported now, get passed on to those who can make good use of them.) Some of these FABs wind up making things like detector diodes (which of course need to be 'large'.) Some for other reasons.
Many ICs destined for use in satellites in space must also have large feature sizes in order to tolerate radiation events. Even some (semi-) higher-end microcontrollers, for example, are still made with large feature sizes for those purposes. (I know because I worked on them, some time ago, as part of writing operating systems that are tolerant to such soft error events.)
I'd recommend contacting a few of the major companies and simply asking them for some good ideas, here. I think you'd find the response welcoming and positive. That's been my experience in the past. If you explain your goals, which seem quite reasonable to me, I'm sure you'd quickly find someone who'd help you think about the best way to develop the curriculum for it and to suggest some good alternatives to consider.
(Besides, if you involved them you would have zero worries about someone later giving you a phone call and asking about why you are exposing their "proprietary" devices in a classroom setting.)
So, to start, you should grab up some 4000 series devices and just "do it." See what you find. If that satisfies your need, you're golden. (I think they will, as there is quite a variety of devices there.)
Second, if you want to expose a microcontroller for classroom demonstration purposes (and not as a difficult assignment, which it probably would be), search out microcontrollers that are still being used for satellite use in space. These are so-called "radiation-hardened" but part of that hardening is about the feature size they use! So look for that, if it is of interest.
Third, contact a few companies and just ask for some ideas and help. I think you'll get it.