Well, as far as I know, none of the popular Android custom ROMs prevent root, though what you found about them not wanting to support you when you run into issu

Well, as far as I know, none of the popular Android custom ROMs prevent root, though what you found about them not wanting to support you when you run into issues is probably somewhat true everywhere. I think the idea is that if something you're doing to get root or after getting root is causing problems, it would be outside the scope of the ROM itself and not their problem. I do still get help in the #LineageOS IRC channel and mention issues in there despite being rooted. I don't mention it unless it seems relevant, but I've seen bug report forms where they specifically ask. Ultimately it's gonna come down to whether they can reproduce it, consider it an issue, and if it's a problem with the ROM or something else. I don't think they intend to be malicious. I used to work in tech support for some enterprise backup software, and if there were signs that the customer's issue was because of their OS or network or someone else's software, that wasn't our problem to fix, and as soon as we could prove that, we could usually close the case. We were only there as support for a specific piece of software, it was their problem, or at least someone else's to keep the general servers and network working. I imagine the volunteer maintainers of LineageOS don't want to help you with things unrelated to what they do either. Have not used GrapheneOS as I've never owned a Pixel, but I imagine it's a similar situation.