It's GPL and available in most linux package managers. Ardour.org ask for payment if you want to download binaries from them, valid for whichever major version number is current, and updates to that major version. That's probably a lot more necessary for Windows and Mac users to get a convenient installer. The payment helps fund the project, so probably well worth paying if it's something you use extensively, even though you don't strictly have to.
There's also Reaper - closed and commercial, with a 60 day free trial period. Some report fewer problems than with ASIO, others more. It probably depends on the interface, and maybe your other hardware.
There seem to be 2 main pain points (other than generally getting used to linux) for people looking to switch:
- Audio interface support. A lot of interfaces don't have linux support at all, or only for the subset of functionality from pure 'class compliant' operation.
- VST support - mostly related to licensing enforcement of commercial VSTs. Most VSTs will run fine via yabridge, but some won't because the emulation isn't quite 100% (might be fixed at some point), and others never will unless the developers change their license enforcement. This is rather like Windows game support via Proton - most are fine, some have a fixable gap in emulation support, but those with kernel mode anti-cheat are stuck unless the developer stops requiring it.