Those are rarely used formats, most of plugins are either in Windows formats like VST, or in Mac formats.
For Mac VSTs and AU there is no (at least, no stable) emulation layer on Linux.
RTAS SDK is proprietary.
"Linux VST" and "Windows VST" are not plugin formats. "VST" is the format, "Windows" and "Linux" are platforms. Do you seriously expect a Linux app to run Windows plugins out-of-the box?
> Most of plugins use Windows VST, i.e. they are distributed as Windows-compatible DLL.
Yes, most (commerical) plugin companies only provide Windows and macOS binaries. But how is that Ardour's fault? You can run Ardour on Windows on macOS. If you really want to use Linux and you need to use Windows-only plugins, you can either run Ardour in Wine or use a plugin bridge such as yabridge.