But some do, or pay those samaritans to, and as a result you can definitely get boards (including this kind of 5-port switches) that run completely supported on a mainline kernel.
My home gateway is a Turris Omnia, supported in mainline for years. I run a plain Debian on it, works great.
Ebay or a licensed reseller (if there are any) should be easier anyways. I remember them requiring a VAT ID for EU customers. I ordered a couple for a small business so that was no problem back then but private individuals are pretty much out of luck.
Higly unlikely, though.
Until then: AVOID!
Wiki: https://www.fw-web.de/dokuwiki/doku.php?id=en:bpi-r2:start