TL;DR: F-Droid isn’t referring to that, but yes, the app requires an API key for a paid service to perform unlimited requests.

Long answer:

When using the expression “non-free”, F-Droid refers to something not being free software, where the term “free” doesn’t refer to its price (free as in beer), but to it giving its users freedom to do what they want to with it (free as in speech).

However, this application in particular relies on a service called AudD, which is a paid service based on the number of API requests done. So while the F-Droid “anti-feature” list doesn’t refer to its price, this app still relies on a paid service and requires an API key upon launch (although it seems you can do a limited number of requests without one).

It has to be added to the command when you want to launch it, or added to the .desktop file so it does so automatically. On KDE, it should be as easy as right clicking it on the start menu and clicking “edit application” on the second tab there should be a command field, where you can add the variable at the beginning.

In case this doesn’t fix it, your alternatives are copying and pasting passwords or, if your main use for it is in a browser, using the official extension.

Using the environment variable QT_QPA_PLATFORM=xcb should do the same thing, but it likely won’t fix your problem. These two methods allow KeepassXC to run on X11, which lets it access other X11 apps (running on XWayland), meaning native Wayland apps still won’t be able to use auto-complete.

There’s probably no way around this for now, as this is due to Wayland’s design, which has stricter keyboard access safety, as opposed to X11 which just let all apps read/use the keyboard all the time.