Motorola Moto Z and Moto G4 will get security updates, but probably not monthly
According to Ars Techinca, Motorola stated that "it's difficult" to roll out software security updates on a monthly basis for all its handsets, and it's more convenient to include them in OS upgrades, or in scheduled maintenance releases. While Motorola assures us that the Moto Z, as well as the Moto G4 series (recently launched in the US), will receive security updates, it simply can't commit to delivering them month by month. Well, we can't say we're surprised: ever since Google started its monthly Android security update program last year, Motorola constantly failed to be on time with these updates.
source: Ars Technica