This must have happened some time in December - but now several DNS providers seem to be blocking all of sslip.io because "IBM Xforce" has put them on a Deny-List.
Thanks @horihel for letting me know; it seems sslip.io has been unblocked by quad9, and I don't quite understand the IBM chart, but if the domain is blocked again please don't hesitate to let me know.
This must have happened some time in December - but now several DNS providers seem to be blocking all of sslip.io because "IBM Xforce" has put them on a Deny-List.
See here for Quad9: https://www.quad9.net/result?url=sslip.io
And here's the source: https://exchange.xforce.ibmcloud.com/url/sslip.io