Does the following sentence imply that X is included in the last release or not?
Since the last release, we have integrated feature X.
The short answer is no. The word since means having occurred in the span of time that starts after an event on up until the present.
The example above would insinuate that there is either a subsequent patch, or that the feature is complete and will be released in the future.
It implies that feature X has been added since the last/previous/June release. (Otherwise, what's the point of mentioning it?)
The long answer is that OP's cited usage of "since" is ambiguous. For example...
I've loved her since I first met her.
...would always be taken to mean that the loving and the first meeting coincided (i.e. - "since" includes the time of the event mentioned). But...
Since Styllou Christofi was executed in 1954, only one other woman has been executed in Britain.
...clearly doesn't include the 1954 execution in the scope of "since".
To provide an unambiguous meaning you'd have to rephrase with something like "As from" (definitely includes the event), "After" (probably doesn't include the event), or "At some time after" (definitely doesn't include the event).