A notable mention is https://ubports.com/en/ which is different from postmarketos in a sense that ubports uses old kernels with heavy patches. That means: good support for things, but difficult future.
PostmarketOS uses the newest kernels and tries to integrate their patches into mainline kernel, so that the reliability is maintained with all kernel developers.
What reason, they are doing this?
In the current world when they are done with a release they have to reconcile their internal trunk (main branch) to the external trunk which is the one that everyone can see. This is because currently google do some development internally while other development is done on the public trunk. This reconciliation process can be annoying due to things like “merge conflicts” (ie. A disagreement between two versions of changes about what the final change should be). By doing all work on the internal trunk it should make this reconciliation process much simpler.