Technically it’s all pretty straightforward. You could bootstrap an i386 only archive with the current packages in eoan and from that point on build only whatever new packages land in eoan, but only build for i386.
I don’t know exactly how many packages actually “need” to be in there, but it’s possible to see what other distros do here, and hand-wavy numbers between ~300 and ~1000 depending on the coverage you’re after. Could be as low as ~100 but you’d have the possibility some 32-bit applications won’t work.
If community maintained it would effectively be like any other repository or ‘ppa’ style archive. It could just follow packages that land in Ubuntu, build them and land them soon after or relatively in sync.
Someone needs to maintain that though, and test it. Who is going to do that? Would users trust it? Would it be allowed to use the Ubuntu trademark? Would it be installed by default? Is it worth having if it’s not installed by default?
If it were hosted by Canonical then users (and customers) would have an expectation of support, and we’re right back where we started. If hosted by the community, who is going to cover the cost of bandwidth (and maintenance) of that repo? Currently Canonical covers that cost, clearly. Repositories are expensive things to run, when nobody is paying at the point of use. Eoan i386 binaries + source is ~150GB in size today. Push out some updates and you have the cost of thousands (millions) of machines hitting that service. There’s a reason we have mirrors and CDNs.
Possible, maybe, viable, unlikely, worthwhile, who knows.