Mostly app related, and all found during the test runs. You absolutely must go through at least two steps of upgrade - 6.4 -> 7.0 -> target, and I suspect your test runs will tell you that 6.4 -> 7.0 -> 7.13 -> 8.5 -> 8.14 is the least problem-prone route.
I don't, not any more.
It used to matter when we had Jira 4 and lower versions of 5, but even then, having a wide gap in the release dates didn't do too much harm - systems would talk to each other, but with one recent release and one ancient one, you'd just see a reduced function set, not a total failure.
Nowadays, I don't think you'll even notice much if you mixed Jira 7.0 to 8.latest with Bitbucket 6.0 to 7.latest and Confluence 6.0 to 7.latest.
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