Awhile back, I was thinking that one pragmatic way to get this viable Linux smartphone moving might be for hobbyists to focus on getting one easily available, affordable device working fully with pure Debian or PostmarketOS (no closed drivers or other modules, and preferably no blobs) and with Purism's Phosh.
Then that would boost contributions to, and demand for, Purism's open source platform/components for Librem 5 (and whatever the successor hardware would be).
If the cheap hardware is something like PinePhone, I'm just going to handwave that maybe this device won't cannibalize much sales of Purism's premium devices, but instead the community investment into the platform will effectively generate much higher net demand for Purism's premium products. With higher volume, Purism could maybe also hit more accessible price points.
If the Purism hardware demand happens, then there may be competing hardware entrants. And they will have to compete partly on being trustworthy and aligned with the interests of the kinds of customer who want to run a non-Apple, non-Google device. Where Purism should have a head start in credibility and goodwill. The new entrants will have to contribute engineer time (possibly: pay community contractors) to getting their device to work well with this platform, and be expected to upstream all of it as open source to the platform mainline, if they want to be attractive to these customers.
(I'm not saying the cheap device has to be PinePhone; that just seemed the most likely one at the time. It could even be something like an older popular Pixel model, with many unlockable-bootloader units available cheap on eBay, for which people are able to assemble/develop open source drivers. Or maybe GrapheneOS will get their own device built, and it can also be used for this non-Android-based open Linux platform.)
I'm behind though: aren't the UIs for mobile Linux still bad? I still can't get the experience I got out of my N900 that had only 256M of RAM, right? Every project I remember to bring the Maemo experience to Linux seemed to wither because there was ho hardware.
(But someone's copy of some of it might have resurfaced now; I haven't looked recently.)
Usually things like this disappear because whoever was paying for hosting for them (company, accounting unit within a company, or some random techie's basement) gets shut down. And maybe no one who had the interest and ability was able to preserve it in time, and archive.org hadn't picked it up. But occasionally, things get deleted with intention to suppress them.
Their core is apparently based on Mer which was a reconstruction of Meego, which was what came after Maemo, merging it with Moblin, IIRC.
It's a bit tenuous, but you might want to look at Sailfish as carrying the torch in that area.