A lot gets written about Optane compatibility, and how it needs a current generation motherboard. But I can't find any reference to whether that's only for Optane directly plugged into RAM slots or used for booting.

When I research it, even Intel's FAQ on Optane just assumes it'll be used as a motherboard NVDIMM, RST chipset cache device or bootable device. It doesn't say a word about data drive usage, and says nothing either way about other formats of XPoint 3D/Optane.

So what I'm trying to find out is a definitive answer whether Optane 2282 cards, and Optane full-size PCIe 3.0 NVMe cards, are compatible with older generations of motherboard.

Also, if 2282 cards are compatible, can they be plugged directly into older motherboards with onboard 2282 slits, or mounted on PCIe with any normal + cheap 2282->PCIe adapter card?

I'd expect them to be as I expect them to have all needed controller/interfacing for both of these buses onboard (other than booting which I don't care about). I'd actually be surprised if they weren't compatible. But I don't want to spend money on one, unless I'm sure.

I think you have the same misunderstanding as in the post
Can Intel Optane memory compensate for less RAM?.
Read in that post why Optane "memory" is not RAM.

See also the video in
How to Install and Manage Intel® Optane™ Memory in Windows® 10 (Basic)
to understand that an M.2 connector is required on the motherboard for Optane
and how to connect the Optane card to the motherboard.

Finally, you will find your answer in the Intel post of
Where to Buy Intel® Optane™ Memory Ready Motherboards.

But please note all the other requirements that are listed in
Intel® Optane™ Memory: Before You Buy, Key Requirements.

(Personally, I don't see why anyone will buy Optane instead of investing in a
real SSD, but that's only my opinion.)

It's now clear that Optane SSDs are 100% compatible with older chipsets and CPUs - guesses about CPU instruction sets are mistaken.

Over a year ago, ServeTheHome.com tested running their entire website on Optane 900p SSDs, to see what difference it made. They reported that

We put copies of the entire STH main site on a dual Intel Xeon E5-2698 V4 system that is identical to another system we have set up in the hosting cluster save that this node had 2x Intel Optane 900p 280GB AICs, 2x Intel DC P3600 1.6TB NVMe AICs, and 2x Intel DC P3700 400GB 2.5″ SSDs.....

We [did] a second day entirely on the Intel Optane 900p 280GB SSDs [and tweeted] "The STH main site is being served via ZFS mirrored Intel Optane 900p NVMe SSDs for a few hours today just to generate some test data.."
— STH (@ServeTheHome) January 2, 2018

What's interesting in this post is a throwaway comment - the "v4" bit. The Xeon E5-2698 v4 is a Broadwell EP CPU (LGA 2011v3). From that alone, it's clear immediately that S.T.H. found Optane SSDs to be compatible enough, and stable enough, with Broadwell era CPUs, to serve their entire site during testing, and to gain very positive impressions on the combination as well.

People have even reported getting Optane to work on old Ivy Bridge 4790k motherboards running Windows 7 - suboptimally for sure, but clearly no fundamental hardware incompatibility.

tl;dr answer:

The limits on using Optane SSD like you'd use any other SSD, are apparently just the need for NVMe support (in the chipset+BIOS/UEFI+driver) and the lack of appropriate physical connectors, both of which are more likely to be missing as boards get older.

Very old versions of NVMe in the BIOS/UEFI might also need to be updated - there are hints about this in a couple of forum posts but no confirmation one way or the other so far.

• (Relevant and not-well-known resource: bios-mods.com is a website that specialises in updating bios firmware modules for NICs, SATA, RST, USB, etc to newer versions, so the boards can be used with newer hardware, drivers or OSes, it could be helpful if stuck)

The main reported issue in older chipsets/CPUs seems to be related to NVMe driver support (in Windows at least), which wasn't yet included in Intel RST at that time.

The requirement for a "latest CPU+chipset" only seems to apply if you want to use Intel's Optane-specific caching/acceleration/NVDIMM functionality, because that needs chipset support. But ordinary SSD use - apparently no issues.