Not making the useage of third party clients a ToS violation.
Sometimes it feels like technology from the past was so bad that, given no alternatives, it interferes with the thing you're wanting to do (Skype/Internet Explorer). Those products can easily be subsumed by a competitor that is a superior product (Discord/Chrome).
But now it feels that because the options for those niches have been completely filled by those services, everyone becomes beholden to them, and in many cases there are undesirable aspects of the services that are not mutually exclusive with them being the most competent product on the market (inability to use Discord 3rd-party clients, Chrome's user tracking, Plex still showing you advertisements after buying a yearly subscription).
It feels like those products became too polished and attractive to users, if there exists a good reason to convince them to use something else, and even if we want to move off them, the network effect inevitably draws some of the holdouts back in (Messenger as a compromise for not using Facebook, sites that are incompatible with non-Chrome browsers). These products now have millions of active users whose behavior can be controlled or manipulated on a whim, for better or worse, if the company decides one day to make changes.
Now I'm starting to get the impression that succeeding in being the best solution in a given market is not necessarily a universal good. So many people were loathing how terrible Skype was for the longest time, and so Discord was founded. Yet, while the experience of participating a chat community is so much easier than in the past, I can't help but thing that so many people are now beholden to a new technological dependency.
What I have to wonder is: if money was not a factor in driving developer decisions, would Discord have allowed third party clients to be used? Preventing automated spam is not a problem unique to Discord.