Wizards Of The Coast Plans To Add "Standard Plus" Format To MTG Arena

My guess is standard plus means, if the deck was considered standard at any point in time, then its legal to play. Perhaps it will mean if your circa 2016 deck you can only fight other 2016 decks. Wait and see.
Do the second set of releases going backwards. 2017, 2016, 2015 along with new content that's moving forward.

What a lacklustre announcement.

Bad enough that you can only progress by winning (there's no compensation for losing a match outside of some potential ticks to your single daily quest and matchmaking itself is a joke, which even the developers have acknowledged given that they're completely rehauling it in the next patch), thus new players seldom stick around long. Without an incentive to play, win or lose, there's scant point in doing so... worse yet, the "win the match or you've wasted your time" pervasive mentality has led to everyone playing the same bloody aggro decks.

What the game NEEDS is a revamp: more daily quests and some long-term progression tracks that reward players for participating regardless of the outcome (say, by playing certain decks or colors, inflicting X amount of damage in a turn, using only CMC spells of XYZ or greater, employing specific cards, etc, all of which would unlock cards of a similar type, thus people who want to play a black vampire deck are rewarded for doing so by earning ever more powerful black vampire cards or cosmetics that fit their style, such as avatars, new card art and deck boxes), larger and custom health-pool matches for people who want to mess about with so-called "jank" cards and not be blown out by aggro in the first three rounds, alongside more casual gameplay modes.

Forget standard: give us Two-Headed Giant or Commander!