I thought I'd try replacing split with explode, which after a couple of tests appears to work now.
There are
11 instances of 'split' (not 'preg_split' , just 'split') to change in
/ includes / functions_autotagger.php , did not notice anything else in the two other files that needed any changes.
"Proof is in the pudding" so to speak so I grabbed a few RSS imports, then manually ran the auto tagger and now it does work with php7... I am running 7.0.10 to be more precise about it.
Pic of a random test RSS thread which then had the auto tagger run on it: