Nobody dares advertise the XML capabilities of their product (which back then everybody did), nobody considers it either hot new thing (like back then) or mature - just obsolete enterprise shit.
It's about as popular now as J2EE, except to people that think "10 years ago" means 1999.
Really, I think you can trace a lot of the "XML is spooky old technology" mindset to the release of HTML 5. That was when XML stopped being directly relevant to the web, though of course it still lives on in many other domains and legacy web apps.
Typically a more primitive (sorry, minimal) format such as JSON is sufficient in which case there's no excuse to overcomplicate things. But sometimes JSON isn't sufficient and people start inventing half baked solutions such as JSON-LD for what is already a solved problem with a mature tech stack.
XSLT remains an elegant and underused solution. Guile even includes built in XML facilities named SXML.
People who wanted to "get shit done" had much better alternatives. XML grew out of hype, corporate management forcing it, and bundling to all kinds of third party products and formats just so they can tick the "have this hot new format support" box.
YAML is just bad. JSON is harder to read for deeply nested structures. TOML and the like don't have enough features.
I’d be very curious what lasting open formats JSON has been used to build.
People upload their podcasts to a platform like Apple Music or Spotify or Substack and co, or to some backend connected to their Wordpress/Ghost/etc) and it spits the RSS behind the scenes, with nobody giving a shit about the XML part.
Might as well declare USSR a huge IT success because people still play Tetris.
What's more, the web standards bodies even abandoned a short-lived XML-hype-era plan to make a new version of HTML based on XML in 2009.
That from this touted to the heavens format a handful of uses remain (some companies still using SOAP, the MS Office monster schemas, RSS, EPUB, and so on) is the very opposite of the adoption it was supposed to have. For those that missed the 90s/early 00s, XML was a hugely hyped format, with enormous corporate adoption between 1999–2005, which deflated totally.
Did you also learned those things too today?
For Web markup, as an industry we tried XHTML (HTML that was strictly XML) for a while, and that didn't stick, and now we have HTML5 which is much more lenient as it doesn't even require closing tags in some cases.
For data exchange, people vastly prefer JSON as an exchange format for its simplicity, or protobuf and friends for their efficiency.
As a configuration format, it has been vastly overtaken by YAML, TOML, and INI, due to their content-forward syntax.
Having said all this I know there are some popular tools that use XML like ClickHouse, Apple's launchd, ROS, etc. but these are relatively niche compared to (e.g.) HTML
XML was definitely popular in the "well used" sense. How popular it was in the "well liked" sense can maybe be up for debate, but it was the best tool for the job at the time for alot of use cases.