For me, integrating features that spans multiple repositories means coordinating changes, multiple PRs, switching branches on many repos to do testing. Quite time consuming. I did use submodules but I find monorepo easier to manage

I don't doubt that it is. Monorepo tools are much better right now. But monorepos don't compose. They don't branch. They don't scale.

My impression is that the world moved on from multirepo to monorepo and I vaguely remember that git submodules have some serious gotchas.

yeah, I dunno how else to say it except that if this feature worked right people would like it