Shamless plug, my ROM DivestOS [2] is available and takes source seriously with most scripts being GPLv3 [3].
Furthermore it appears that they are mixing incompatible license HOSTS sources for their blocker. And according to their FAQ the are shipping devices with the bootloader left unlocked, just like /e/.
How awful.
And to your last point, NetGuard is available [4].
[0] https://iode.tech/en/iode-solution/ [1] https://gitlab.com/iode/ota [2] https://divestos.org/ [3] https://gitlab.com/divested-mobile [4] https://github.com/M66B/NetGuard
However on select devices you can successfully relock the bootloader with DivestOS installed, as it is properly signed [0] and enables verity on supported devices.
Other alternatives that support relocking are GrapheneOS [1], CalyxOS [2], #!OS [3], and RattlesnakeOS [4]. They too do not sell devices.
[0] https://github.com/Divested-Mobile/DivestOS-Build/blob/maste... [1] https://grapheneos.org/ [2] https://calyxos.org/ [3] https://github.com/hashbang/os [4] https://github.com/dan-v/rattlesnakeos-stack
Then you can allow or deny domains or apply blocklists.