User bots (including hacked clients) are officially banned by the TOS, which addresses that concern.

The only acceptable API usage is via bots that server owners choose to invite. And while it might be legally OK (if the bot's own TOS says it), I promise no server owner is expecting an invited bot to slurp up every message for use in a data set, whether that be for academic purposes or a potential stalking/"dirt" database.

I highly doubt this is the most ethical instance of data collection.