I saw this also a couple of weeks ago when downloading 21.02.1 images. But since I download imagebuilder and verify them with the gpg public key downloaded when 21.02 branch was released I continued with the download.
I think that the situation has changed a lot from year 2017 (the first thread), as currently both master and 21.02 already default for https with the opkg package downloads by the router itself. Routers have https support by default since August 2020 (due to WPA3 readiness).
opkg switch to https was also done in August 2020 by
Support for 19.07 will soon end, and all further releases should support https.
Also desktop browsing in general is nowadays largely https based, so I see no special reason why we should stick to promoting http downloads any more.