Postmodernism is a term used by critics, to group together a range of disparate thinkers, many of whom rejected the label, like Foucault. A good student of postmodern thought, should be very wary of how far generalisations about postmodernism stretch.
The assumption that there is one true reality, and one ultimate narrative about it, is revealed by postmodern thinkers as a power-grab. The tool of imperialist cultures, autocrats, and theocratic states. Moving beyond that doesn't mean 'anything goes' relativism, it means we have to argue things out, pursuade each other of interpretations, and rely on other means to sort fact from fiction than who has a monopoly on force.
Postmodern thinkers have had a deep connection to literary criticism, analysing the works of leading thinkers celebrated in a culture, for insights that are not in plain view. You might look at this answer about postmodern thought in art criticism, for how a single perspective gives way to analysis with a toolbox of ideas: Does postmodernism in art criticism collapse into relativism? What's its merit? Far from diminishing or relativising what art is, this is about looking at how art speaks to different people, what it does to people and how. The art ceases to be isolated, interpreted for the masses by cultural high-priests, and becomes something for everyone to chew over, if they can talk interestingly, and draw out insights.
You might find this answer interesting, about modern developments in postmodern thought, in postcolonial theory: Need help with this paper on epistemic justice The totalising assumption that justice is constituted of one set of ideas known by everyone, is unpicked by thinking about epistemic justice. Access to social discourse, like art, plays, literature, films, festivals, is part of marginalised groups defining their own identity themselves, and determining their own ideas about what justice is. That's not relativism, but instead accepting that human thought is fundamentally community-led, rather than just imposed by powerful individuals - who can never be as creative and dynamic as groups, and typically just picked from options supplied by others anyway (see the history of state religions).
Now, I would specifically criticise Kuhn, for claiming truth cannot be separated power - if that were true cheap propaganda would win against good science. But nuclear weapons were invented, and Nazi & Imperial Japanese poor quality physics left them less fully engaged with reality, as well as defeated. I am saying this as prologue to the postmodern definition of science: Science is, what scientists do. Discussed in detail here: Philosophical assumptions underlying science It is the same shift. There is no golden tablet of ultimate axioms: science started as a community, as a culture, bound by certain aims, and is in a constant dynamic process across many fields, to refine, update, and improve, how science is done. To say then that science is consigned to being totally relative and 'anything goes', is to completely miss how science is a living culture, defined by it's participants, and ultimately if they are pursuaded to change their epistemology, or standards of statistical methods, or that replication of key studies based on small participant numbers must be replicated before being relied on, then science itself has changed.
Saying that means science can be changed arbitrarily and without limits, is to miss how society and culture work. Durkheim is invaluable in this, in showing how social cohesion comes from putting values beyond question, so you challenge the values and you challenge the culture - but in science this is an active, dynamic process of scrutiny, not a holy book that cannot be rewritten and reinterpreted.
I see the 'postmodernism is pure relativism' perspective as a view only held by people who haven't studied postmodern thinkers. Analytic philosophy has sought to model itself on mathematics and science. Continental thought has remained engaged with culture, as giving insight into human nature and reality, and personally I feel it is crucial that philosophy continue to take up that ground, working to make our lives intelligible to ourselves, not withdrawing to a playground of formal sentences in predicate logic barely of interest even to a tiny niche of academics.
Discourse, critique and pursuasion, don't undermine firm foundations for thought, they are essential to locating them.