I have no problem with you modifying the code or using my mod as a framework, but I would ask that you restore the LICENSE.md and NOTICE file you removed.

You have not asked for permission and no permission has been granted to remove those files or license.

You have removed the license from your mod. I repeated this change with the Fixed version. For what reason do you want to keep the license only in the Fixed version?

The version that you forked this mod from used the CC 4.0 BY-NC, just because a later version of the mod has a new license doesn't mean this mod gets to retroactively pick whatever license you want.
You are welcome to fork 2.2.0 and recreate your mod with 2.2.0 of Rampant that has been licensed as GPL, if you want to change the license to GPL.

Unfortunately, the changes in the fixed version are very poorly suited for Rampant 2.0 (and 3.0), and it is quite time-consuming to develop anew.

In addition, in the near future there were plans to release Rampant Fixed 2.0 working with any number of biter/player forces, taking into account diplomacy - but as a result, the data structure became slightly different. Perhaps it's for the best that this moment (license) has become clear now.

However, I had suspicions that over time there might be disagreements with you as the author of Rampant. I will return the license in the next release. (~ 5 days later, minor technical improvements )