I've been researching dumbphones lately and wanted to share about the developments I've learned. I'll be writing from an European perspective.
I am omitting Android since I wasn't interested in it. Android Go is discontinued, if you care.

CloudFone

This is an addon to the barebones OS manufacturers add to their phones. Such OS' are e.g. HMD (formely Nokia) S30+ or other Mocor RTOS based systtems.
This addon is an "app" within the OS that's a browser which offloads the rendering to another server. It works similar to the Puffin browser.

The advantage here is that the underlying browser engine is ran and updated on the server.
This helps avoid the KaiOS situation: KaiOS v2 (the last version in Europe) uses Firefox 48 (current version is 137).
CloudFone could be running the latest stable Chromium even on an old device, as long as the rendering server is updated to that version. The remote server rendering is obviously more powerful than what the little feature phone can normally do.

You can see how it works here: https://youtu.be/coaLnA7Twl4?t=295

The disadvantage here is that those apps do not work offline - you need to connect to the server over the Internet to render them.
If the underyling rendering server is ever shut down, you lose all your apps and your phone is back to being a dumb-dumbphone.
It seems like you don't have control over what apps are available and which are not. These could be rug-pulled at any moment. There are some rumors on /r/dumbphones about a WhatsApp CloudFone app which would be big.
Some of the apps are something you wouldn't want on a dumbphone, like tiktok or yt shorts.

The trick is that the firmware versions with CloudFone enabled are only offered to phones in India. The only way to get these firmware versions is to download a custom firmware from the Russian 4pda.to forums.
This custom firmware seems to be available for Nokia 3210 4G 2024 or Nokia 220 4G. A more powerful option would be HMD 110 4G 2024 since it has 128 MB RAM, but I couldn't find the CloudFone enabled firmware for it.

I get that this approach is not acceptable to the freedom-oriented, tech-savvy demographic on Lemmy, but it looks like this is where the mainstreaim is heading right now.

The downside of the non-KaiOS devices is that they normally don't support WiFi and thus can't serve as a mobile hotspot. There are devices like itel R60+ which can, however, but I have no idea which website to import it from.

KaiOS

The latest KaiOS version on devices sold in Europe is KaiOS v2.5.x. The latest available outside Europe is 3.1 (?). There's supposedly KaiOS v4 in the works. People say it's dead.

KaiOS is just not an European thing - this is balantly obvious if you look at HMD's "Barbie phone" - it uses KaiOS 3.1 in the US version, but in Europe, it uses the basic HMD S30+ OS.

There's a KaiOS jailbreaking community. See https://wiki.bananahackers.net/en/devices for supported devices. Apps you can install with the jailbreak are here: https://store.bananahackers.net/.
I've seen an XMPP client and a Matrix one too.

I've been only considering devices with a USB-C port and available in Europe and what I've found is Blackview N1000 (somewhat easily available on Allegro in Poland, has USB-C and is jailbreakable according to bananahackers wiki, but it supposedly resets itself on long +20m phone calls), Gigaset GL7 (USB-C, unknown if jailbreakable, available only if you buy secondhand from someone), myPhone UP smart LTE (USB-C, non jailbreakable), Maxcom MK281 (microusb, not known if jailbreakable, can buy secondhand only). Note that some of those aren't jailbreakable according to the bananahackers table above.

You could also import an US KaiOS v3/v4 (TCL Flip 4 is KaiOS v4, US only) phone, but the overlap in LTE bands is only on band 7 (I think?), meaning it'd only have reception in cities. There's someone that imported an US Nokia 2780 and reports it works in Italy on /r/dumbphones.

KaiOS devices mostly can serve as a mobile hotspot, which is nice.

postmarketOS

Phones that run KaiOS out of the factory normally have 0.5 GB of RAM, meaning they can boot Linux. See https://wiki.postmarketos.org/wiki/Category:Feature_Phone
The newest device in this table is the NA Nokia 2780 released in 2022. The feature support tables seems not to support calls.

SoCs

The "Feature phone SoCs" section seems to be gone from the Unisoc website. The Wikipedia SoC Unisoc table lists e.g. T107 but doesn't list the newer T127 or T157 (supports 5G and only ever used on Asian feature phones)