@mwilliams I did mean Exchange SE linked to O365 in terms of "subscription service", not hybrid setup. I really hate the "you can't buy your software anymore" paradigm, on top of the "Servers needs to phone to O365 console regulary" scenario.
Anyway, I tend get away from O365 as much as I can nowadays, for those exact sovereign questions.
But you're right, I genuinely thought EWS was sunsetted even on Exchange SE, so my reasoning about not using EWS for archiving in the future was wrong. Thanks.