For some reason IPFS does not return correct last-modified HTTP header if the file accessed using IPFS hash /ipfs/something, but it does return it if you use ipns or domain with _dnslink
This is seen on ipfs.io docs:
$ curl -I https://docs.ipfs.io/introduction/usage/index.html
…
last-modified: Mon, 04 Feb 2019 11:03:48 GMT
$ dig txt _dnslink.docs.ipfs.io +short
"dnslink=/ipfs/QmVwAhBZSWCCs1UKMUUzDaT5gZM6bj64rjcn2vyhb2Dbgt"
$ curl -I https://ipfs.io/ipfs/QmVwAhBZSWCCs1UKMUUzDaT5gZM6bj64rjcn2vyhb2Dbgt/index.html
…
last-modified: Thu, 01 Jan 1970 00:00:01 GMT
Is this a bug or a feature?