If you're using Qt applications purely in the open source world (e.g. KDE on some Linux distro) there's no new news here.
A lot of small businesses use Qt for free under the LGPL license (allowing them to keep their application closed-source as long as the user is free to replace the Qt dynamic libraries). For the past several years many new modules are commercial or GPL only, preventing their use by LGPL users. For example, QtWayland, QML Virtual Keyboard, WebGL, Qt Quick 3D.
Some companies would bite the bullet and pay for Qt their exorbitant fee, then let the license expire and keep using the old version. As you say, this will no longer be possible.
I find Qt 6's license policy awful, I simply don't believe in "pay us forever even if you're happy with an old version". Early on Qt 6 even had a draconian commercial policy that wouldn't allow you to distribute an old binary after if your license was expired and you stopped new development, thankfully they walked this back.
- The initial versions of LTS point releases (so e.g. something like a 5.15.0) are open source. The commercial-only begins a few point releases in, say with something like a 5.15.3.
- Due to the KDE Free Qt Foundation agreement, also those initially commercial-only point releases do become open source eventually, at a max 12 month delay.
This was indeed very controversial when it was done for the first time with Qt 5.15.*, since that is the last major version if Qt 5, which effectively meant no same-day open source maintenance releases for Qt 5 anymore after some point. Within the Qt 6 release series it's less of a big deal, in that by the time a 6.x.y is released commercial-only, usually the next 6.x is already around the corner.