Slack – How to Hide Your Timezone

No. You Slack user and profile is always part of the individual Slack team / workspace. Slack does not support global users.

As I understand that is part of Slack's security architecture, where it's not possible to see other Slack teams unless you have been explicitly invited.

The answer, sadly, is it cannot be done, at least not at this time of writing.

I wrote to Slack support directly who advise this is not possible. I then asked if they could at least add support so people's default timezone is picked up from their device. They advised this is a commonly requested feature and is on their roadmap.

So, I am answering my own question as the official answer is this is not possible at this time - disappointing but true.