>Putting a +1/+1 counter on Ajani's Pridemate is no longer a 'may' ability'. Note: This is official errata. The new oracle text is "Whenever you gain life, put a +1/+1 counter on Ajani's Pridemate."
From the Latest Arena Patchnotes (JAN 17 - 0.11.00.00): https://forums.mtgarena.com/forums/threads/45109
The fact that they have the errata doesn't mean that they caused it
I mean, there's a good chance that it was prompted by Arena, MTGO, or both, but the fact that they mention it in their own release notes doesn't imply that
Them calling it out in the patch notes makes me think that Arena had some effect on the change. The Pridemate "issue" has had a decently vocal group asking for changes, especially since the "reason" behind it getting a may trigger isn't really relevant anymore (I think they changed it in 2016 or something?)
the change to whether people need to remind their opponents about mandatory triggers: http://magic.tcgplayer.com/db/article.asp?ID=10802
