Google Safe Browsing looks to be have been built without consideration for open-source or self-hosted software.

IMO this has nothing to do with user safety. The harder they make it to move away from their products by making using alternatives difficult, the more money they make. Even if this only adds a fraction of a fraction of a percent to their profit it’s something they will do. The old implied social contract of businesses providing value to the community as a whole in addition to making a profit is long gone.