Yes, every small guy has to do that, to avoid being wiped out by legal action if a bug results in someone suffering loss or damage, despite his or her best efforts. If you are giving software away free, it is entirely unreasonable to be held to be liable. Developers already give a great deal of time and intellectual effort, with often no direct reward, except the immense satisfaction of doing a job well.
What is unacceptable is that the Big Bully Boys of the industry (pick whichever you hate most, M$, Apple, Oracle, SAP.....) are also able to hide behind similar get-out clauses, whereas the manufacturers of just about any other retail product are liable for the consequences of their products malfunctioning. Someone, I think it may have been the dangerous incompetent who ran M$ at the time, once claimed that it was because software was not yet a mature industry. That was more a reflection on the maturity of the individuals concerned, himself in particular, because, unknown to him (I don't think he is capable of understanding), techniques for producing rock solid, reliable code, and the necessary hardware to run it reliably, are well-known and understood, but rarely practiced. Nevertheless such techniques are only within the scope of large teams, with some capital to invest, and the bulk of software that is written is either just quick hacks to satisfy the internal needs of a business, or open source for public distribution. The amount of expensive commercial software that is written, and can economically support the procedures needed to give it high reliability, is most likely well under half of the total, whether measured by lines of code, programmers hours or installed copies.
Nevertheless, the "small" developers can, and do, apply good practice to their coding in many instances, and turn out stuff that is as good as any commercial product, if not better. Long may that continue. People should always feel the need to do things well, rather than just adequately, it provides more satisfaction in the end, and everyone wins. Visibility of source code helps, of course.
The developer is to be congratulated and thanked for running a successful product for all those years. Closing down now is a sign of success, not failure. The successors to m0n0wall live on, much like how UNIX is virtually dead (thanks to a certain Mr. McBride who sued the world and lost), but one of its successors (Linux) is found in the majority of smartphones (all Androids and some more) and umpteen other places, being the numerically dominant full OS when all computing devices are considered. (OK, not very significant on the desktop, yet...). And look what the other successors, mostly the xBSD family, are doing. Apple products, many routers and NAS boxes, etc. Solaris lives on, despite almost the whole world hating, with good cause, its current owner, Oracle. So UNIX is all but gone (the remnant of the business lives on, still). Was it a failure? Of course not. Likewise m0n0wall was, and will always remain, an outstanding success.