I think it is now considered a sin against humanity to create more Flash videos. 
Anyways, it is possible in Shotcut:
- Format = swf
- Codec > Codec = Default for format
- Codec > B frames = 0
- Audio > Sample rate = 44100
- Audio > Codec = Default for format
That worked for me. If it does not for you, then view the failed export job’s log and read the reason why. That is what I had to do to learn to change B frames and audio sample rate.