Plutonium

Assuming you're actually telling the honest truth here, this probably happened because you didn't switch to LAN mode before launching the game and proceeded to inject a mod - editing memory.

I have no way to check logs to see if you actually did test said mods in a custom games or in a public server, as I am not a developer or admin, however it is a bit concerning that you are new to Plutonium, and decided to start testing mods only after 2 matches. Your best bet: if you plan on injecting mods furthermore, disable the anticheat and play on LAN mode by clicking the alternate button - the down arrow beside the green "PLAY" button.

Hopefully this helps your future experiences with experimenting PRIVATELY with mods enabled.

Not a moderator here, however, in future circumstances please provide a little backstory on what happened before you got banned.

In the event that you injected a modification, or tried editing memory, the active anti-cheat picked up on it, triggering an automatic ban. If this was a ban from an available server on Plutonium, you may have committed an act that was against the server owner's set rules (i.e. allocating more than 10 class slots [CaC exploit], continuous toxicity, or something along the lines) in which they disagreed with your actions. If this happened globally - or in other words within the Plutonium client itself, as stated before you attempted to edit memory in some form.

If you were curious on testing modifications PRIVATELY, take strong consideration into playing on LAN Mode. This disables the anti-cheat entirely and lets you play on a your custom server. In order to enable LAN Mode, you need to click the arrow beside the green "PLAY" button in the Plutonium launcher. Notice the color changes to yellow, with text indicating that the anti-cheat is disabled.

--For further reference, attached is two screenshots of what you need to click on to switch to LAN Mode.