Well, I checked it briefly using a simple workflow in my 7.6 environment, and indeed the execution doesn't seem to stop at breakpoints in the legacy client.
Not an intended removal of functionality; looks like something got broken during migration/implementation of the debugging functionality in the new HTML client
You should be able to debug the same workflow in the new client, though.
Thanks for this Ilian. Will you file a PR/introduce a fix for HF1 or should I open an SR with technical support?-
If it is not a problem for you, I'd suggest to open a SR with tech support.