I’ve played with the same idea of making an Android client but I would never build a product on that because I know the limitations on my side.
As a company you are 100% allowed to break 3rd party client when they don’t have an agreement with you. It’s your product after all. Heck even with an agreement APIs don’t support old versions.
Why do you think they don't want you to run a jailbreak? It's to protect the walled garden. If you can install apps other than their store, that's lost revenue. They claim security blah blah, but it's removing mouths from the teet. So, it has everything to do with the walled garden. How does that not make sense to you?
It's similar to how OpenAI uses "safety" to make sure their LLMs don't get them in hot water, and PlayStation uses "safety" to make sure their consoles do not become associated with piracy and make publishers think twice.
This kind of "safety" is about business interests. :) Some companies can say it openly that they wish to protect their business, as fundamentally there is nothing wrong with that. Others can't as that will bode poorly for their monopoly status and they will suffer (overdue) legal repercussions. So it becomes "safety".
Notice how companies that argue against user freedom for "safety" are always in circumstances where bringing up business interests behind "safety" won't bode well.