Approximately when we can expect it to online ?
Soon™
In all seriousness: I don't know... I'm working on it, theymos is working on it, each of us have different styles and different ideas about how it should even be approached, let alone how it should function.
It's not currently high on my to-do list, because:
(*) There's still some thinking that needs to happen around whether or not (and how) the exported data should be encrypted.
(*) There's still some thinking that needs to happen around whether or not (and how) the exported data should be signed.
(*) I recently bumped into a 2020
poll that asked "Do you want a utility to save all your PMs?", and of the 25 respondents, 11 said "Yes" and 14 said "No".
Like a lot of things that either theymos or I could do/add, writing the code is not the bottleneck. It's the thinking through of all the implications, and the careful weighing of all the different approaches that takes time.
I also think there's a good chance that few people will actually use this, which will leave the forum with most of its current and future data-safety burden, anyway. (And, ignoring both the issue of message-containing e-mailed PM notifications, which I think trades sender-privacy for receiver-convenience in a not-well-thought-out way, and the issue of Cloudflare's MITM, which I very much hope won't be something that the forum has to live with forever, I also feel like any data-unburdening that does result from this will come at the cost of that data being duplicated and spread around in ways that it mostly isn't right now.)
Finally, when I look at this whole problem through the lenses of the issues that underpin it (like improving PM search, and mitigating against data breaches), I have other ideas that I think would sum to something much more effective. (But, I think it's unlikely that those ideas will be pursued; I only mention them to help explain why it's difficult for me to prioritize this one: In general, the older I get, the more disinclined I am to work in directions that don't make sense to me.)