Depends on your definition of "now", but I'd say they've been very common for at least the past 15 years (you could pencil CC Sabathia down for one in the spring when he was still in Cleveland), especially in spring training or guys on rehab after a lot of downtime, and seemingly more with pitchers. Sometimes it's also just terminology. I'm pretty sure I'd never heard of an oblique strain 30 years ago, but I'm also sure they were happening and they were just calling them another name, like a strained midsection or torso or some such thing.