After testing this for a couple of hours, it looks like it works ok with Confluence 7.4.0 (at least for simple tables). With a few mentions:
- it doesn't seem to work with old tables. So if you have old tables where you'd like to use this feature, you might need to extract the data from them and add it to a new table.
- it doesn't work in Edit or Preview modes (not sure why). You need to Save changes. So if you wanna experiment with this feature, my suggestion would be to create a test page.
- doesn't work anymore if your table contains merged cells. Interestingly enough, Google Docs' "freeze row" feature works just fine with the exact same table. So...maybe we can have that, too? That would be awesome. Or maybe there's a workaround for that and I haven't found it yet.
Suggestion: if it's not supposed to work with merged cells (though, again, Google seems to have done it somehow), please, implement a warning/notification, something to let the user know that if he's gonna merge cells, this functionality will be disabled.
Anyway, I hope this comment helps other people. It was definitely frustrating for me not to have it working for so long. 
This is not working correctly. I have settings as follows and they are not freezing columns and rows in both view and edit mode. Please fix. Creating large tables are USELESS without this feature..