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I just viewed this question where Ajani's Pridemate was mentioned. I noticed a small but significant difference; on all printings, including the recent Core 2019 set, its card text says

Whenever you gain life, you may put a +1/+1 counter on Ajani's Pridemate.

Yet its official Gatherer card text omits the "you may" part:

Whenever you gain life, put a +1/+1 counter on Ajani's Pridemate.

Why/how was this done? I can remember some rule changes happening soon after the card was released (e.g. Walking Atlas which became an artifact creature) but not on a card which was already in play for 8 years; Ajani's Pridemate made its first appearance in 2011.

The ability was originally optional, and it was made mandatory with the release of Ravnica Allegiance. The Ravnica Allegiance Oracle changes update bulletin explains the reasoning behind both the original choice and the errata. The short answer is that changes in tournament policy, increased popularity of digital Magic implementations, and renewed popularity of the card made the errata desirable and worthwhile. The full explanation is quoted below:

Ajani's Pridemate was originally printed under a tournament policy where missing any trigger, no matter how beneficial, always resulted in a Warning. Multiple Warnings would upgrade into a Game Loss. To avoid the problem, many cards with strictly beneficial triggers had those triggers made optional—it doesn't count as "missing" a trigger if it's a "may," you just chose not to do the thing.

Then trigger policy changed. Nowadays, missing beneficial triggers doesn't get you a penalty, you just don't get the benefit (there's some nuance to this, but I'm leaving it simple here). We've left the cards that received "may" under the old policy alone under the general Oracle policy of minimizing functional changes, so Ajani's Pridemate was printed in Core Set 2019 with its "may."

Ajani's Pridemate has since become heavily played in a popular Standard deck, and that deck's inclusion in the upcoming Challenger Deck series became the place to print the card with errata to remove the option, making the card look and read better on paper and improving the life of digital Magic players everywhere. There isn't an Oracle update scheduled for small supplementary products like Challenger Decks, and given the options of adding one or changing the card early, digital was the deciding factor and we're releasing the card's updated wording a little early.

It is a bit complicated but Wizards actually released a short article about the changes.

ORACLE CHANGES: AJANI'S PRIDE-MAY-TE (FUNCTIONAL)

To summarize it is a combination of a couple of reasons:

#1

Due to changes to the way missing Beneficial Triggers are handled in tournament settings, a major reason for including the "May" doesn't exist anymore. Previously if there was a mandatory trigger both players would be responsible for remembering the trigger and if a mandatory trigger was missed it would require judge action to repair the gamestate.

Since the change, that is no longer the case. Nowadays if a "Beneficial" (gain life, creature gets bigger etc.) trigger is missed, too bad for the player that missed it.

#2
Ajani's Pridemate is a popular card in the online Magic Arena game. It is also included in the "Tutorial" decks that are provided for free to players when they first get the game. This means that not only is the creature encountered fairly frequently Online, but it is also featured in most players first impression of the game (and for some non-paper players maybe even their first experience with magic).

Normally this wouldn't be that bad but the "May" trigger is quite obnoxious for pridemate in digital as each trigger requires player input, so something like swinging with a board of lifelinkers may require the player to click accept 20 times per combat. It can slow down the game quite heavily. Removing the "May" allows the triggers to auto-resolve with no player interaction (something that is not possible with "may" triggers currently).

#3

It makes the card read slightly better in digital (and paper when it is re-printed).

The how is that it will be printed with the now wording in Challenger Deck coming out in 2019 and an announcement on official magic website (
https://magic.wizards.com/en/articles/archive/news/ravnica-allegiance-oracle-changes-2019-01-22 )

The why boils down to (digital) user experience issues vs tournament policy.

In the past, during the tournaments with higher REL (rules enforcement levels) one could receive a game or match loss for missing any trigger, even a purely beneficial one, if it wasn't optional.

These rules caused the creators to make many beneficial triggers optional. While working good for tournament rules, it caused some annoyance for digital players - especially Arena ones as at the time of writing one cannot automate the response to the trigger (which is possible in MTGO).

Since then the tournament rules were updated to allow for lesser penalties for missed beneficial triggers (i.e. opponent choosing if the trigger happens). Given that magic R&D team decided to errata the Pridemate to make it more convenient for digital players.