there is plenty open source software, that you can buy. There are many modes:

• you buy the support (redhead)
• you buy the long-term-support (ubuntu)
• you pay for backports to old releases (keycloak iirc)
• there is a open source version, and you can pay for enterprise features and hosting (gitlab)
• there is an open source version, and you pay for customization (star office, iirc)

and my personal favorit:

• you pay a random developer to submit pullrequests for bugs that are relevant to you

This is all true, but it’s interesting how people often forget another simple option: the software commercial - it is simply sold on some website/store. Just like you can buy the game Mindustry on Steam, but it’s Libre Software and even though you can get the build for free on GitHub and its itch.io page, people still pay for the Steam version. I wonder why people forget about this option, since it’s probably the simplest one.

Of course, Steam is a proprietary, unethical platform, so I’m just using it as an example - I’m not saying we should sell there.

The best example that comes to mind is Ardour, a FOSS digital audio workstation that charges for binaries. Their FAQ says

If you want the convenience of using our ready-to-run version and/or for support from Ardour developers and experienced users, we ask that you pay something for this.

If you don’t want to pay for a ready-to-run version, then you’ll need to get the source code and build it yourself. We do not provide assistance with this process and particularly on Windows and macOS this can be challenging and take a long time. Also, for Windows, there are no instructions.

It’s a bit mixed in with the “pay for support” model, since you’re basically on your own if you build it yourself. Which probably makes support a lot easier, since there are fewer supported configurations. This seems like a pretty workable model for something as complex as Ardour, but idk how well it would work for simpler projects.

think it also helps that Ardor is used directly by individual users, and its proprietary competitors are often quite expensive. If you’re making something that already has a lot of “free as in beer” competition, this may be harder to pull off.