Angular 6 and all future main releases will be supported for 18 months.

As for backwards compatibility, as was described in the presentation on the new renderer they are working on, 600 apps at Google are built on Angular, so there is a great deal of internal pressure to make sure backwards compatibility is a constant consideration.

Along those lines, one of the big tooling features in this release is the upgrade command in the CLI, which can allow packages to self update themselves across your codebase (i.e. if there's a breaking API change, they can rewrite that API call accordingly automatically during package upgrades). This is new, so it's yet to be determined how common and painless the usage will be.