MarsSeed [1] filed a deletion request for boost-libs-171-opt [2]:

Defunct legacy boost 1.71 package, source 404, and there are other problems.

Uploaded in 2020 and never fixed.

There is another boost 1.71 package in AUR which is better maintained.

So I recommend deletion, as a few commenters did in 2020 and 2021.

[1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/boost-libs-171-opt/

For the records, the other boost 1.71 pkgbuild on AUR (boost171) wasn't good (old version in place of current version, break all packages using repo's package) and have been recently deleted too [PRQ#43576]