I'm like 99.9% sure he isn't leaving. Even if he is, a lack of social media talk about his job and response from the EE twitter page shouldn't lead you to that conclusion; they'd very likely not respond to random fans - if DS or someone were to get in touch, maybe....
EastEnders actors are entitled to time off, and Sam Strike has been on-screen as Johnny consistently all year. Actors get approx 4 weeks maximum off (characters can be off-screen for longer i.e. Denise hasn't been seen since 30/09 and those cases are often down to special circumstance/requested longer leave).
There've been times when Johnny's not been seen for a few weeks but Sam will have been working consistently. The show is filmed out of sequence and the Carter's have had A LOT of scenes, so even if Johnny's been off-screen, Sam will still have been working regularly due to the bulk of the job.
I'd wager his recent time-off has been due time off. It's a little concerning that it seems to have been around the Xmas filming but again, I'd tack a lack of social media interaction with a pinch of salt. No reason to worry.
Johnny's getting a storyline again soon anyway