Although it does say in the 6.46 docs that the 18 turn limit is now removed, for the first time today, I've had the game terminate on me after 20 turns, with the "Donations are welcome" message.
So it looks like even version 6.46 is still shareware. Sorry about this, because I reported earlier that it looked like the turn limit had been removed (I thought it had).
It must be that I had never got to 20 turns whilst playing it over the last few weeks without even realising it. (It had been crashing on me under WinUAE)
Still don't know why it states in the docs that the limit has been removed though!