Okay, I upgraded to "2018-04-22b", which had only some very smal changes in two files.
But just as assumed, the warning messages stemming from the Data plugin persist.
I am not very familiar with PHP, especially what has changed in the new version, so I don't know how to fix it now.
But as already mentioned, I think the new PHP version is just checking certain conditions more strictly and giving warning advice, even though no real error happens. So it is probably nothing to worry much for the time being.
I have opened an
issue on the GitHub page 
for the Data plugin. Maybe someone will have an idea on what to change to satisfy PHP 7.3.
