I strongly second the recommendation to version customer-facing APIs. You should never be releasing backwards-incompatible changes without giving customers the option to upgrade on their own timelines (within reason - its unreasonable to expect you to support an older API for a decade; 3 months is an absolute minimum, though, and I'd recommend more like 18-36 months for enterprise customers; and there can be exceptions for changes that address severe security or data loss defects.)
And yes, at the very least, email communication is appreciated. Even if you're versioning - if you have a customer using an outdated API, you should be warning them that it's deprecated and then again a couple times leading up to EOL.
Support has since informed us that the announcement was in the monthly newsletters we don't read because they're usually all marketing materials.
This also allows you to put hooks in to track users of the old api version so that additional reach out/communication to the slow adopters can be had.