The non-existence or non-availability of recursive features, like an issue being parent of an issue, or in Slack a thread being started inside a thread and so on, to me usually indicates, that the developers of the tool are afraid of recursion, and have never learned how to implement this easily in a database, used a programming language that easily deals with recursion, and don't know how to deal with it in other languages. They tend to think "it's too complicated". It's usually just a silly lazy excuse, but here we are, with tons of inflexible shitty tools.