There is no such thing as an absolute with even open source code. Sure, it's open to use, but LL has every right to list who they deem are legitimate TPVs, and/or subset viewers, as well as grant or revoke the rights of viewers to be used on Second Life. Emerald Viewer is one case where a TPV was once part of the official list, then taken off, given a cease and desist, and blocked from access to SL.
It would take some time to re-package it for the mobile environment. However, Android is essentially a fork of Linux developed by Google, and i-phone shares common language with Apple's PC - both of which many official devs are familiar with and have made working viewers for already. It's not about the code itself being impossible to run, but it is about how you package the code and refine it to work on phones that had things like different resolutions than home PCs, and how to get the hardware dependencies to work nicely with the code. Not really something that would take over a decade to do, and something that ought to have embarrassed LL that a small dev created Lumiya almost more than a decade before they got around to making an official mobile viewer, assuming they are still working on it and will ever get it out of the testing stage.
It is a term that is understood what it means, and has a certain legal precedence that has not been utilized yet. The old copyright laws that could range up to 70 years before free use kicks in made sense back when you only expected royalties of public works to only really matter to the author and publisher of said works, for as long as they are alive to receive a check. But that's not how things work in an era when mega corporations are built off the back of dead, zombified corporations that currently exist only in name under the umbrella of the parent corporation - the mega corporation that owns it. Abandonware is that which is left in a publishing grave yard (or junk yard) or previous things that the original authors no longer hold the rights to, and thus do not get paid any more by its distribution, or lack thereof. It's also what that which the company that holds the rights to the product either don't know they have, or don't care to do anything with. At this point, when a product or code is abandoned, the question could be on whether it is necessarily piracy to use or distribute it? It's a grey area of the law, and is starting to get hashed out a bit, particularly now with streaming apps, question over who owns the rights to the content and its distribution, and what obligation is there to the user of a streaming app, who paid for said content. It's not a question that will go away, and currently entangles a variety of things beyond just LL and Second Life open source source code. How a variety of cases are decided will set real life precedence over who has ownership of what, and what rights users, developers, publishers, and platforms have.