Using email efficiently requires training and discipline, being able to:

-- at a quick glance, quickly "triage" big lists of unread messages, not be put off by several hundreds messages and grab the phone

-- reply shortly and to the point (being native to the culture and language helps but is not a given anymore)

-- master art of attracting attention of very busy people of various kinds, to get them open at least the email and read some

-- not to turn email into slack in disguise, check if the question already has answer available somewhere else, and not become nervous if the other side doesn't reply in a second

-- master email client sufficiently to configure pre-sorting messages, and trust it's action

I'd say environment where email works as a reliable channel for top-importance communication (such as goals and prios), probably already doesn't need OKRs.