That server has very restrictive TLS settings and cannot be accessed by older web browsers or curl installations on older operating systems which would otherwise be able to use your program:
$ /usr/bin/curl -IL https://www.lcdf.org/gifsicle/gifsicle-1.95.tar.gz
curl: (60) SSL certificate problem: Invalid certificate chain
More details here: http://curl.haxx.se/docs/sslcerts.html
curl performs SSL certificate verification by default, using a "bundle"
of Certificate Authority (CA) public keys (CA certs). If the default
bundle file isn't adequate, you can specify an alternate file
using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
the bundle, the certificate verification probably failed due to a
problem with the certificate (it might be expired, or the name might
not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
the -k (or --insecure) option.
If you can adjust the server to be less restrictive, great. Otherwise, or in addition, please create a GitHub release for each tag and attach the corresponding distfile to each release, at least for the current release and all future releases but ideally also for all past releases.
You currently host your distfiles on your web site at https://www.lcdf.org/gifsicle/
That server has very restrictive TLS settings and cannot be accessed by older web browsers or curl installations on older operating systems which would otherwise be able to use your program:
If you can adjust the server to be less restrictive, great. Otherwise, or in addition, please create a GitHub release for each tag and attach the corresponding distfile to each release, at least for the current release and all future releases but ideally also for all past releases.