According to our current policy, from community consensus in a top-voted answer in the meta FAQ post, What's the policy on abbreviating titles of Movies & TV Shows?
If the original title fits into the limit, it is the only objective criterium by which one can go and thus should be captured as accurately as possible. And if it doesn't fit it is really a (maybe subjective) movie by movie decision driven by common sense, reading esthetics and archival purposes. And meta appendages should only be used when actually necessary.
Since the original and official title of the movie is "Zack Snyder's Justice League" (sources: HBO, IMDb, Rotten Tomatoes), the tag should be zack-snyders-justice-league (which fits the tag character limit).
From the same answer linked above:
Tag changes that extend tags by meta information in order to resolve ambiguities, like this
-tv or the common year-appendages, should be postponed until those ambiguities actually arise.
justice-league-2021 has an unnecessary meta year appendage, "2021" (which is not part of the official movie title), and according to our policy, should not be the main tag (since there is no ambiguity). It should be made a synonym of zack-snyders-justice-league instead.