Here, here! Yes, hosts have every right to refuse to play with any player, for whatever reason. However, they don't have a right to defame a player and tell others to not join that player based on lies.
It seems that ProGUI provides an empirical advantage. It's not unfair because it's freely available. However, currently, people don't have a clear indication of whether another player is using it. I like the Zero-K philosophy of allowing software development for automation of certain aspects of the game. But, some people have a strong opinion against using ProGUI or joining games where others are using it. I think that hosts, and ideally other players, should have a right to know when a modification that provides an advantage is being used by another player. That way a host can make an informed decision about whether the player is in compliance with the rules they have set for the game they're hosting. It's especially relevant in competitive games like rated matches, tournaments, and apparently the average game involving some of the more, shall we say, high strung players.
So, I would support adding a feature where the mods that a player has enabled that offer a potential objective advantage in the game, are disclosed to the host.
It would also be nice to have a feature where a host can deny users to connect when they have certain mods enabled.
This feature would be easily bypassed by anyone who decides to actually cheat by modifying the source code of the game or renaming their mod. The only counter is to make 0ad closed source and use an anti-cheat system like Easy Anti-Cheat. That won't be happening, so ultimately we rely on the honor system.
Yet, a mod that offers an advantage is potentially detectable by watching replays. So, if a player cheats a rule about which mods are allowed by a hoster in a match and uses a disallowed mod anyway then evidence of it will be evidence of actually cheating.
As far as timing for implementing such a feature, that depends on who supports its development. Calling all junior C++ and JavaScript developers: talk to me about software development plans.