From the 14.2 Slackware-HOWTO
The architecture is also recorded in the non-SMP kernel names. e.g. <mirror>/slackware/slackware-14.2/slackware/a/kernel-generic-4.4.14-i586-1.txz

RHEL is now 64-bit only (and Oracle Linux too if I recall) but most of the others that you mentioned run on 32-bit x86: CentOS, Fedora, openSUSE and Ubuntu. (CentOS is not very useful though because there aren't extra repositories like EPEL for x86.) Personally I use a small laptop with an Atom CPU that doesn't support 64 bits (I'm postponing buying a new one for some time).

CentOS 7 is 64-bit only (because based on RHEL 7), the older (but still supported) 6.x versions will run on 32-bits.
The same goes for openSUSE, the Leap (42.x and 15.0) releases are available in 64-bit versions only.
And although ubuntu, kubuntu and lubuntu still support 32-bits platforms, the install media (since 17.10 Artful Aardvark in the case of ubuntu) come as x86_64 iso's only (see releases.ubuntu.com. I quote from the 17.10 page:
As for Fedora, the 29 release also only offers a 64-bit iso.

So I'll stay with my original remarks!

I agree that support for 32-bit CPUs in these distributions is somewhat reduced compared to 64-bit CPUs but it's an overstatement to say they are 64-bit only. I've personally run CentOS 7 x86 (AltArch) and Ubuntu 18.04 in a VM on a 32-bit-only laptop, as for Fedora and openSUSE the first link on the websites to download an ISO shows images for both 64-bit and 32-bit (Fedora 29 Workstation and openSUSE Tumbleweed).

Ubuntu provides a cloud image and a netinstall CD for x86. You're right that the desktop and server installation disks are 64-bit only, but other flavors such as Xububtu/Lubuntu have both 64-bit and 32-bit installation media.

CentOS 7 has AltArch releases for x86 and arm32, on the website click Alternative downloads to see them.

That depends a bit on "which VM", i.e. VirtualBox can emulate a 64-bit processor on a 32-bit system (Pentium-4 or later, with no x86_64 yet) and I did run CentOS-64 in VB on an openSUSE 32-bits system some years ago.
The other way is even more easy as all 64-bits cpu's can still run 32-bit code.

I must admit I didn't look at Tumbleweed (the rolling release) but to the Leap releases only. I used to administer student PC's with SLED and used openSUSE to develop the image used for those PC's (both 32- and 64-bit).

Again, I didn't look at those - I learned something new again - I only mirror and seed (BitTorrent) the official iso's for CentOS and ubuntu/kubuntu and they were 64-bit only.

So thanks for expanding my knowledge, I am NOT too old to learn.

PS: our university file server does not carry the AltArch versions:
the centos/7.6.1810/isos directory only had a x86_64 subdir, so that explains a bit the confusion. I normally do not work with the "real sites" as we carry mirrors of all distributions we, or our students, are using. Partial mirrors, it now turns out.

Let's leave it at that, we're far enough off-topic already.