Thanks to Babykim for sharing this, JamesWhite said in the last developer stream the only forbidden modification is the aim assist. Consequently, everything else is allowed. In particular, not forbidden are:
- Rainbow Mod: Shows the place of shell fall on the water surface, while the salvo is still in flight. Makes it very easy to dodge. The same exists for torpedoes.
- X-Ray Mod: Always shows the outline of all ships, even if they are behind islands.
A probable reason for not banning these mods is the inability to reliably detect them.