This page is mostly for end users that may have questions/ accusations about you app that uses the in-game API
There is no short answer to this question as it all depends on what you are using the app for and how it is getting that data. Lets talk about what will get you banned:
- Doing anything in unspoken rules
- Pretending to use the API to cheat / doing anything in the unspoken rules
- Cheating
Beyond this there is a grey area with no real "good" way of using the API. If Riot really wanted, it's not very hard for them to see that these API calls are not from the game itself.
Riot view of the API officially is also quite confusing. On one side they say it's not supported in any way and they cannot guarentee that you won't be banned for using it. On the other side they say that any app that uses the API / reads game files and memory is bannable. Then what makes thing like Overwolf which literally reads game memory not bannable? However, using an oversight to justify your app is not a good idea.
It all depends on what it is doing. You have to think about wether it makes sense for something to be not allowed. Does having an external app to manage your skins or crosshairs seem like cheating? No, of course not. Use your head, if you dont trust something, dont use it. You can try asking Riot support but the best they will supply is the exact same resposnse citing that it is unofficial and that they cannot guarantee it's safety.