PSL/PSA has a history going back to late 1967/early 1968. It has a long and distinguished applied research foundation, probably the most extensive research project ever, in the modelling of requirements.
That solid research base was followed by a turbulent period of an attempted break-out into the commercial world. As a consequence of several decisions that, with the benefit of hindsight, were unfortunate, PSL/PSA as a separate product commercially available, ceased to be available. It is an absolutely classic example of how not to do a university commercial spinout to try to exploit new intellectual property!
Since PSL/PSA ceased to be available, there have been various attempts at new approaches to requirements modelling. The most widely used replacement is probably UML. However, one fundamental reason why life is being breathed back into PSL is precisely because of a widely held belief that UML has not yet demonstrated a capability that is a match for the capabilities of PSL/PSA. There is no doubt that the core of PSL/PSA needs some adjustment, but it has stood the test of time.
Therefore, PSL/PSA will be brought back to life with various incarnations. That is a substantial part of this website. The subsections of this history, discuss the past, present, and future in more detail.