Earlier quoted context omitted.
OneDrive has been around for 10 years and we're one of the best known companies for backwards compatibility and keeping things far past their prime (Hotmail/Outlook.com, for instance, which has been out for 21 years, albeit only 20 under the company.) What makes you think it's going to be surprise-discontinued?
Probably more detail than I can really articulate, but I've lately perceived a shift in how Microsoft wants to support legacy and maybe getting more aggressive in taking a stance in eliminating some. Right now Microsoft seems really high on cloud, particularly as a means to make Office a subscription product (which helps with legacy support, as in the long run, there are no legacy versions to support in this model).…
Edit: For me, after Apple actually decided to integrate iCloud into macOS, I felt more secure about it staying, but before that it definitely felt like a pet project that might be abandoned. I still don't fully trust it with my documents, but since I have an iPhone, I have well over 15GB of photos and videos uploaded that I'd like to see stay for a while. :)