They’re pure garbage.
Really, all they mean is that some Bozo somewhere registered a Twitter account while claiming to be a pro-Russian rebel in the Ukraine, and then used said account to post something boasting about shooting down a plane. And then later removed the post.
Maybe it was an actual rebel who actually shot down the plane and then realized it was bad PR to boast about it. Maybe. Or maybe it was someone pulling a joke who decided the joke was in bad taste and should be removed. Or maybe it was someone on the pro-Ukraine side who was posting “black propaganda” designed to make the enemy look bad. Or someone connected with the US military-industrial complex who wants a new cold war with Russia. Or one of dozens of other plausible possibilities.
If, that is, such a post was even made in the first place. Curiously, I haven’t seen so much as a purported screen shot (itself an easy thing to fake) of it. Just an assertion that it was there and then later vanished.
Unless some far better evidence comes up, the only prudent response to such reports is to wave them off as the unsubstantiated rumors that they are.