I will echo what others have said here, if only to just reinforce it, about my being glad as well to hear that the forum will not be deleted. That's awesome! To be fair, it's been a while since I have interacted with either of the two GTD platforms (that's life), so I haven't been keeping up with where things stood.
Though, at some point, there were some changes (I assume) on here (the GTD forums) that had the default profile pictures being replaced with question marks (which is not very inviting). At that point, I figured the forum wasn't long for this world; I de-prioritized at that point. Glad to hear that I am mistaken and that it's indeed sticking around.
As for the Mighty Networks side, honestly, I haven't really done anything on there since registering and briefly browsing around. As I said before, the style and experience that MN is going for didn't really appeal to me. That's fine and if it appeals to others, that's cool too. However, I didn't want to dismiss it (as it's the direction David Co. is going) without trying to give it a chance at least.
Now, I am not sure if it's just me, but I have _always_ had trouble just accessing the MN site at all. I figured out it was because of my ad-blockers and DNS restrictions for privacy, anti-tracking, and so forth. I did a temporary test with them disabled and the site worked. Now I am not sure if it's something on the MN platform itself or a configuration for GTD's community specifically on MN but the site has always just blank paged for me with protections on. In fairness, I didn't submit a bug report or anything like that to MN or David Co., so the issue could be fixed. That's on me. I just don't have the time or interest in playing tech support/doing free QA for sites though (specifically MN).
Furthermore, unfortunately, anti-tracking and privacy/ad-blocking always being on is a hard red line for me. I trust David's company and the great folks he has. However, I _absolutely_ unequivocally _refuse_ to disable any of my blockers or protections to use any site or service. I have been operating this way for over 12 years now. The abuse of users, invasion of privacy, and ocean of forced advertisements online is _so_ pervasive these days that it has even been officially recommended by the FBI for years to not use the internet _without_ ad-blockers and privacy protections.
Again, to be clear, I am not saying anything negative about David's team or the decisions that they have made (I completely understand their perspective around the burden of maintaining a custom distribution of XenForo), but I don't trust Mighty Networks. I have never even heard of them before GTD switched to them. Like others here, I prioritize my privacy and explicit decision to choose what I share and with whom I share it with and under what terms. I don't use social media at all either for similar reasons.
If MN ever fixes their site to comply with and be usable without tracking or ads or such, then great and I might use them then. Though as it stands, I don't see my interacting with it being very likely at all. Until then, I will use and interact with this platform for as long as it remains.
Not super helpful or insightful but figured I'd mention my take on things all the same.