For regular usage the ARM Compiler 6 is not free to use and a licence have to be purchased. MbedStudio have an exception and in a combination of MbedStudio & AC6 it is possible free to use.
However the license needs to be renewed, that is usually done via updates.
- GCC will be working anyway I think.
- You can always use Keil Studio Cloud
- You can setup your own settings in VS-Code
BTW KeilStudioCloud is much better than MbedStudio now.
BR, Jan