You're definitely safe writing your own apps in it for well into the foreseeable future — there's not only way too much app code out there for it to be deprecat

You're definitely safe writing your own apps in it for well into the foreseeable future — there's not only way too much app code out there for it to be deprecated soon, but the vast majority of the system and bundled apps are in Objective-C. I'd expect Objective-C jobs, though, to decline in number over time as Swift jobs rise. If you're doing it professionally, not learning Swift is going to increasingly limit your options, perhaps pretty rapidly — a few months ago, I got turned down for a job in part because of lack of Swift experience. Ended up landing a different job where we're writing a Swift app and step 1 for everyone on the team was "learn Swift".