Hi Michael,
Unfortunately I don't have an SAP BW 7.4 system, otherwise I would be keen to test it myself.
I agree with you that it looks like the BEx 3.x runtime functionality might still be present. I cannot judge to which extend you are still able to use it.
From a maintenance and support point-of-view, I think SAP's direction is clear (and not new since release 7.4). Migrate existing BEx 3.x content to 7.x. Maybe the most safe option is to do that prior to the upgrade.
I think it could be wise to ask for SAP's standpoint re. supported or not.You can raise an SAP Support message for that. If you would do that, please keep us informed since it's useful information to share.
Thanks in advance.
Best regards,
Sander