@aparcar
Ok, I found it. The IPv6 address for sysupgrade.openwrt.org site is bad.

Using GNU wget -4 ... works fine, wget -6 ... hangs forever (well, I killed it after three minutes, so close enough to forever).

Also

$ nslookup sysupgrade.openwrt.org
Non-authoritative answer:
sysupgrade.openwrt.org  canonical name = asu-01.infra.openwrt.org
Name:   asu-01.infra.openwrt.org
Address: 206.81.21.114

Non-authoritative answer:
sysupgrade.openwrt.org  canonical name = asu-01.infra.openwrt.org
Name:   asu-01.infra.openwrt.org
Address: 2a03:b0c0:3:d0::1574:1

$ ping 206.81.21.114
PING 206.81.21.114 (206.81.21.114): 56 data bytes
64 bytes from 206.81.21.114: seq=0 ttl=47 time=175.577 ms
64 bytes from 206.81.21.114: seq=1 ttl=47 time=172.185 ms

$ ping 2a03:b0c0:3:d0::1574:1
PING 2a03:b0c0:3:d0::1574:1 (2a03:b0c0:3:d0::1574:1): 56 data bytes
< hangs forever >
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