The future needs at least one completely Open Source, Open Hardware ONT... ideally several...
A Google search at this point in time seems to fail to locate even one...
The next best thing (a step in that direction) might be open source firmware for existing proprietary ONT's, for which I found the following links for people who are apparently attempting getting something like that working:
"Has anyone tried making custom firmware for your ONT?":
https://broadband.forum/threads/has-anyone-tried-making-cust...
"Build for Nokia G-2425G-A":
https://forum.openwrt.org/t/build-for-nokia-g-2425g-a/106936
Anyway, the future needs a completely Open Source, Open Hardware ONT...