The most relevant, detailed thread about this type of question is here:
If a browser is very popular, and if it uses a similar or identical dictionary to either Chrome or Safari, my prediction is that KM would add support for it. But look at this chart of browser popularity on MacOS:
If you are wondering why Firefox isn't supported in KM, when it has 11% of all OS X users, that's because "Firefox doesn't support AppleScript." This statement comes from PeterNLewis and it implies that if Firefox did support AppleScript, then KM would add actions for it. I infer that the threshold for support is somewhere below 11%.
The Brave Browser is very similar to Chrome, in terms of AppleScript support, so there are threads on this forum that give advice on how to make it work. But since Brave is under 0.3% of all MacOS browsers used by users, that's probably why it doesn't have actions in KM specifically to support it.