U.S. municipal bonds and municipal bond funds, such as Vanguard's VTEB, are U.S. income tax exempt for everyone, including U.S. persons. The yields reflect that tax advantage, of course, so foreign investors might be less interested in them. But they are available to U.S. and non-U.S. persons alike.
I'd say "definitely less interested" rather than "might be less interested". Right now, AAA munis yield less than US treasuries at every point on the yield curve (though it's pretty close to parity out in the long end, give or take about 10bps of yield). But if you're a US taxpayer, that might still be a pretty sweet deal.
Quick muni-bond-splainer for anyone who hasn't run across them before: US tax law has a special carve-out for "municipal bonds", that is, bonds issued by state and local authorities. The interest on those bonds is exempt from US income tax; and, if you pay tax in the state where the bonds are issued, they're exempt from state tax as well.
For example, leaving aside credit risk, a US taxpayer would rather own a 30yr Florida muni bond yielding 2.95% than a 30-year US treasury yielding 3.13%. Why? Because if you're paying a common-or-garden-variety 28% marginal tax rate, that 3.13% yield on the US treasury becomes about 2.25% after tax; but the Florida bond's interest payments are tax-free, so you get the full 2.95%.
I live and pay tax in California, so it's even more of a no-brainer for me. In California, there's a 10-ish percent state tax on top of the 28-ish percent federal marginal tax rate; so if I bought a 30-year treasury yielding 3.13%, I'd lose 38% of that to tax, leaving me with 1.94% yield after tax. But a 30-year California state government muni bond yields 3.7% tax free. No contest. (And yep, I own California munis as part of my bond allocation.)
(Obvious disclaimer: California's economy is pretty swingy, and hugely leveraged to the tech sector. As recently as 2009, everyone was convinced California was going to default because its tax revenues had collapsed so badly, and that sort of fear will almost certainly happen again sometime in the next thirty years. So, these are not a rock-solid bet; like any other bond, they have some risk, and the price will inevitably move around.)
But if you're not a US taxpayer, there's not really a great reason to own munis, because you can't benefit from the tax break; you don't pay tax on regular US treasuries, so in our previous example you get the full 3.13% on the US treasuries. Why would you buy a Florida bond yielding 2.95% instead?
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That said, weirdly enough, there is apparently some offshore demand for US munis, even though offshore buyers don't get any tax advantages from them! A little bit of googling around says that about 4% of the US muni bond market is held by offshore investors: it's a mix of Asian life-insurance companies buying California (which they like because it's got a relatively high yield for a AA-minus credit, though personally I think they're a bit blind to California's economic volatility, see above), and hedge funds and vulture investors chucking it around in the cesspit that is the Puerto Rico muni market.