I’ve heard rumors that DNS records are also sometimes used in some steganography-type communications. Great way of passing small messages in a ubiquitous and innocuous system, unlikely to be blocked or raise eyebrows by accessing.
gemini -> gemini://gemi.dev, it has geminipedia, a web to gemini converter reading sites over gemini at great speeds.
irc -> servers from https://bitlbee.org will allow upon connecting to a registered IRC account to several protocols in the server. For instance, XMPP users will appear as IRC users and groupchat can be created as IRC channels. Ditto with Mastodon, Discord...
mail/usenet -> well, except for big attachments and news binaries (free NNTP servers will just serve text) once you used something like mbsync/msmtp to store your IMAP mail locally and send email ondemand (and ditto with Usenet with slrnpull doing the same exact same task for pushing your writtings and pulling down new articles) everything would just work slower, but usable enough as it can be just batch-uploaded/downloaded overnight.
Iodine it's really great for open but paid wifi services behind portals, such as some hotels, airports...
It won't give you broadband speeds but you can at least chat with people, read some blogs or news at https://lite.cnn.com or https://text.npr.org or get some classic from Gutenberg. That's better than nothing.
I've never had great luck getting iodine running anywhere. The one and only success I've had was on an aircraft where, after numerous attempts at different things, the best I could do is connect to an SMTP server and send an email manually.
If you consider information theory, when something has states, you can store data in any system that has multiple states, which means you can store data in any system.
The placement of coffee cups on a table can be used to encode data.
At that point, only your audience needs to know that data is there.
