Let me explain real quick:

semlar used a custome made client for warframe in order to gather the data send in the puplic chats of warframe (region, recruit, tradechats of all regions etc.). So u were able to either check what rivens people were trying to sell in trade chat (roughly 48h timewindow got tracked and not the entire history!) on: https://semlar.com/trade/ or u could check out a live webversion of all these chats on: https://10o.io/rivenhunter/.

Not semlar himself got banned (or I dont think so atleast) but his bot he used to gather the data.

There u have the reason why the bann is more or less understandable, DE doesnt allow 3rd party software to read there game-data as stated in there EULA, only if they allowed it, and semlar is clearly breaking those rules even if it is for a good cause. So we gotta wait now and see if DE wants him to continue with this or not.

I dont endorse the bann of this tool or am happy about it, however it is fair and I bet semlar knew it was coming sooner or later. I hope DE and semlar sort something out asap :)