Post agile is a thing. Everybody is pretending to be agile these days. So, the word has become utterly meaningless. Every bank, insurerer, etc. is doing agile. And they are just as boring, stupid, and ineffective as 20 years ago. Government IT projects still go spectacularly wrong but they all pray to the church of Agile now.
When the agile manifesto was signed (yes old enough to remember), this was not the case. Agile was a new thing then. People were doing all sorts of stuff at the time and confusing processes and modeling techniques and requirements engineering methodology. Universities taught waterfall then (mostly because academics have no clue how development actually works). Rational Unified Process was something pimped by a company specializing in UML tooling! They ended up in the hands of IBM; probably the least agile company in existence at the time. Blue suits/white shirts were very much still a thing then. RUP was considered modern in the late nineties.
UML itself perpetuated the dogmas of waterfall, which was to first do detailed designs (using UML) after doing requirements specifications and before doing implementation work and before testing would commence. Automated testing was not a thing at the time. With rational unified they added a thin layer of iterative; meaning you got to do waterfall multiple times in a project. Rational's premise was that this required tools, lots of tools. Very complex tools that required lots of consulting. This is why IBM bought them.
Iterative development is of course almost as old as waterfall. The original paper by Royce on waterfall is actually a pretty good read but was soon complemented by papers on spiral and iterative development. Feedback loops are a good thing; every engineer knows this.
What the agile manifesto accomplished was that the UML bubble was burst. Having a lot of design documentation slows you down when iterating and makes it hard to do that. When people figured out that the added value of this typically incomplete and out of date documentation was questionable and that iterating was a good thing the result was that UML became a thing for whiteboards and from there an entirely optional thing. Same with requirements documentation, which was a bit of a black art to begin with. With agile people figured out that it's much easier to specify small deltas of what you want changed then the whole thing up front. Issue trackers empowered this. Bugzilla was the first popular one that got a lot of traction. They turned requirements into a way of working.
In a post agile world, everybody uses similarly capable tools. Typically git, some issue tracker, async commnunication tools like irc or slack, etc. I exclude email here; relative to 20 years ago, I spend a lot less time looking at email. It's rapidly disappearing from my life at least. Async communication is at odds with meetings and empower having distributed teams. The open source world was always distributed and never relied on meetings. They were an early adopted of asynchronous tools.
With post agile people are discovering that the added value of meetings is questionable. Agile initially replaced tools with structured ways to organize teams. An unintended side effect was lots of meetings. Meetings are inherently synchronous in both time and (usually) space. They require heads in a room at a specific moment. Video conferencing sucks and remote attendees are typically at a huge disadvantage in such meetings. So meetings are an obstacle for having remote teams.
With post agile, people are keeping some of the tools but are abandoning meetings. This is similarly liberating as saying goodbye to convoluted out of date UML diagrams, crappy requirements specifications, and the glacial pace of waterfall style development. Other post agile trends are continuous deployment: if it's ready, ship it now, not after the next retrospective in two weeks. And it's key enabler: continuous integration: aka. automatically assess whether you are fit to ship right now and do that after every small change. The former eliminates release management as a role and the latter relieves product managers from having to manually test and approve releases. That cuts down on meetings and eliminates Sprints as a necessity and allows you to iterate in hours instead of weeks. With sprints out of the way, you can delete the associated meetings. Standups are not practical in a distributed team, so those go as well. Post agile is about asynchronous communication and work distribution and getting rid of synchronization bottlenecks in processes (aka meetings).