sbbs.el

Looking further into this, I think this might be a schemebbs issue, or at lease an issue with the configuration of textboard.org. 

I ran 

$ curl -I https://bbs.jp.net/sexp/mona/10
HTTP/1.1 200 OK
Server: nginx/1.18.0
Date: Sun, 31 May 2020 12:28:44 GMT
Content-Type: text/x-scheme
Content-Length: 6175
Last-Modified: Fri, 29 May 2020 18:22:58 GMT
Connection: keep-alive
ETag: "5ed15302-181f"
Accept-Ranges: bytes

where the Content-Type is just "text/x-scheme", without any encoding data. I'm not sure how url.el handles this, but in eww the same post gets rendered correctly.