That was mandatory as of or is mandatory as of

Solution 1:

It's just writing, and technical regulatory writing at that, so nothing about this would have the gravitas of literature grammar.

The mood of the sentence is imperative ("Learn about...") up until that dependent relative clause "that was mandatory." As you point out, this is further complicated by the semantic implications of "mandatory," i.e., it might as well be conjugated in the present tense if it is still applicable, and referred to with a simple copula. As a matter of historical record, though, "was mandatory" seems like a broken construction (e.g., "mandatorized") in the passive voice of a latin mandare or "to mandate,", e.g., "It was mandated January 2011."

As it reads now, it reads that, if you were to check the "status" of NEBC2017 in December 2018, it was not mandatory, but in January 2019, it was mandatory.

Me personally I would write something like:

Learn about commercial wireless lighting controls that help satisfy [the new] NEBC2017 (mandated January 1, 2019).