John McIntyre has identified the "split verb rule" as "The Dumbest Rule in the AP Stylebook" (You Don't Say, 4/9/2016):
[A]s you look through Garner, Fowler, MWDEU, and language authorities whom you reckon by the dozens on the subject of the split infinitive, you will not find them treating what the AP Stylebook imagines is a problem with splitting a compound verb. That is because placing an adverb between the auxiliary verb and main verb is perfectly idiomatic English, and has been so for half a dozen centuries and more. The authorities do not identify a problem there. If the split infinitive bugaboo is nonsense, than the split compound bugaboo is a fortiori nonsense. John Bremner dismisses it in Words on Words: “Those who would ban splitting a compound verb are even more antediluvian than antisplitinfinitive troglodytes.”
Since many reporters habitually observe this imaginary rule, I have to conclude that it is a linguistic artifice perpetrated by journalism schools, with the editors of the Associated Press Stylebook supplying aid and comfort.
It is time, past time, for this stylebook entry to go to a crossroads and lie down.
In my opinion, John is being unfair to troglodytes here: the "split verb rule", like the prejudices against sentence-initial conjunctions and singular they, is a relatively recent pop-prescriptivist invention, not an attempt to preserve an ancient principle.