I really want the world to move on from monorepos to multirepos. Git submodules set multirepos back by 10 years, but they still make more sense. The are composable!
My impression is that the world moved on from multirepo to monorepo and I vaguely remember that git submodules have some serious gotchas.
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