Matt Mullenweg, co-founder of WordPress and CEO of Automattic announced on Reddit that WP Engine initiated legal action against WordPress, Automattic, and Mullenweg himself. Mullenweg wrote that WordPress is countersuing.
WP Engine is a leading managed WordPress host provider that Mullenweg alleges is violating the WordPress trademark.
The software is free, but it looks like the trademark is not. So WordPress bans WP engine from some WordPress stuff b/c they aren’t technically WordPress. In other words, they’re free to use (and change) the software, but they can’t (or, rather, shouldn’t) use the name—according to WordPress. WP sues for usage anyway after they are barred from some event or something, but now WordPress is suing back, turning an unofficial dispute to a legal one.
But they didn’t use the WordPress trademark to brand themselves. They used “WP”, which WordPress has dedicated to the public domain.
Similarly to Nextcloud. If you host an instance and remove preinstalled apps, you are obliged to also remove the Nextcloud branding.