There are a lot of proposals, rfcs, discussions, everything is really fragmented. Can we have a tracking issue that will track the progress and collect those (including links to useful threads from internals)?
A tracking issue is only created once a proposal is accepted.
For partial borrows, there have been a lot of proposals, so it's difficult to summarize them. Try searching for partial borrows on IRLO.
I believe that view types is the most promising proposal so far, but I'd prefer view types to be nominal rather than structural.
I agree, but the topic is important for many and people need a place to get the information. "Searching" irlo will not give you the state. Yes, formally this not a proposal, but people need central tracking anyway. This is a known topic with people constantly participating, but it is very fragmented, which could be partially attributed to the lack of any centralized tracking.
Maybe not a "tracking issue" formally, but some other kind? Maybe create a new kind?
The best cases I've seen for this usually have the author of a new RFC collect all these disparate pieces of discussion, summarize them, include resolutions (or lack thereof)... it is indeed a lot of work. The worst such cases typically involve a vague suggestion, and then sometimes the exact same beats of call/response from a previous discussion.
Which is to say I don't know if I have anything concrete to add, but I welcome the advent of your metaissue tracker if it can be made.