Abandoned plugins tend to be a little extra-interesting, because they tend to be honest dork-ware.

This category of software includes a disproportionate ratio of projects that were begun by someone who encountered an unfilled need, or opportunity, but who lacked well-developed skills with either programming in general, or coding within the WordPress environment in particular, or commonly both. And the time to continue a committment ... really, the dominant factor in this dynamic.

The dork-inventory is notably rich in efforts to address a narrow niche which will not lend itself to profit or fame. It is just not unusual to go looking for a nicely-defined functionality coded-up as a plugin, and find that it just plain doesn't exist ... or if it does (uh-huh/uh-oh) it's old, unloved, and clunky.

The original 'idea' behind such plugins tends to be free of the various 'ulterior' motivating factors that become the drivers of plugin development, within other common social contexts.

So dorks tend to create plugins under more-idealistic motivations, but their implementation tends to be junk. But they also tend to use programming-techniques that are easier for other low-brow coders to follow; and which don't necessarily have to be "poor" technique. I can actually see & tell what they're doing, code-wise ... unlike, unfortunately, some 'proper' plugins.

Kinda-obviously, this situation has been recognize for a long time, since targeted efforts to at least save, preserve, archive these 'nice try - no cigar' plugins have been around for a long time.

I myself keep a /plugins-arc directory in /wp-content, with several hundred entries, some long-gone from Extend. Periodically, I log out, rename /plugins to /plugins-off, and rename /plugins-arc to /plugins, log back in and perform an Update. Sorting the directory by Date shows what has remained untouched, and what is regularly Updating.

[Populate /plugins-arc by first deactivating the titles to be archived, in Admin. Then login out and move each plugin directory to the /arc directory. Watch that your MySQL doesn't get too loaded with abandoned Tables.]

I know a lot of this stuff I archive is trash (even originally, and now badly outdated, to-boot), but it's interesting trash ... for which there is often in fact still not a good replacement, in the official WordPress repository.

There are some strong upsides to simple dork-ware. It's worth paying attention to, and saving.