That's exactly the problem, NerdTools 2024-01-06 updated lftp to the latest version. As per the Slackware-current changelog:
n/lftp-4.9.2-x86_64-4.txz: Rebuilt.
Recompiled against openssl-3.0.8.
Which affects both the libssl and libcrypto dependencies:
# ldd -d /usr/bin/lftp
linux-vdso.so.1 (0x00007fff1e3f0000)
libexpat.so.1 => /usr/lib64/libexpat.so.1 (0x00001512ad731000)
libssl.so.3 => not found
libz.so.1 => /lib64/libz.so.1 (0x00001512ad717000)
libcrypto.so.3 => not found
libreadline.so.8 => /usr/lib64/libreadline.so.8 (0x00001512ad6bf000)
libtinfo.so.6 => /lib64/libtinfo.so.6 (0x00001512ad68d000)
libidn2.so.0 => /usr/lib64/libidn2.so.0 (0x00001512ad659000)
libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00001512ad436000)
libm.so.6 => /lib64/libm.so.6 (0x00001512ad350000)
libgcc_s.so.1 => /usr/lib64/libgcc_s.so.1 (0x00001512ad32f000)
libc.so.6 => /lib64/libc.so.6 (0x00001512ad138000)
libunistring.so.5 => /usr/lib64/libunistring.so.5 (0x00001512acf81000)
/lib64/ld-linux-x86-64.so.2 (0x00001512ad76a000)
If I install lftp-4.9.2-x86_64-3.txz over top after boot, lftp works again. Disabling lftp in NerdTools altogether and dropping the previous version in /boot/custom/usr/share/packages/ would be another workaround.