Is this a recently updated OH? Maybe you need to force a refresh of the browser?
Yes, it is, but I made a reboot after update from openHAB itself and also refreshed my browser.
I just tested with another browser in privacy mode and still no HTTP block 
For completeness I also installed openhab-addons but no change. HTTP section still missing.
I know that I can use the classic way like here: HTTP Post using blockly
Or just run a bash script with curl and get the result … Have to think about it.
PS: HTTP binding is installed, shouldn’t be the problem.