FWIW, this specific feature - what they are now calling sub-issues - is actually better described as a framework for modeling proper parent-child relationships in their system, which is something quite hard to get right, mainly because it has to work somehow with the existing issues feature set. People building this feature from scratch (e.g. Linear) have it trivial to solve, because they didn't have any backwards compatibility issue to worry about. This is, of course, absolutely Github's fault for not designing it to accomodate such a thing easily in the first place, but the people who built this feature are probably not the same people who made that original decision.

They've been working on some version of this feature for several years now in various iterations. I believe this is either their third or fourth attempt to get it right - I was trialing a beta of some previous iteration of it a few years ago and it was incomplete/not fully well thought out which must be why they dropped it. I'd trust the feature here at least to be decent now, because of how many attempts they've had at it.

But yeah if I was a company like Zenhub I would be probably a bit worried at this announcement since it is almost inevitable that this specific feature is going to be enough for people to no longer need third party management of issues. I know in a previous company I worked for that specific feature (proper parent-child relationships) was the reason they used Zenhub, and same for my current company using Linear.