Unfortunately the owner doesn't make themselves available to be contacted for anything at this point. They were more involved years ago.
As far as who owns the data, everything mentioned above is community-owned at least. So if anything it'd be up to individual community members, I'd think, to be OK with it. Since the forum is fairly dead, many of those original members aren't even active either.
Depends on where the data is stored, if the owner of the domain lives in the EU or a similar jurisdiction the data will be under Database rights for 15 years. There is no such thing as community owned. Each post might be owned by the person who posted it but often that is not the case.
EDIT: so of course you should dump the data, wget -r and dump it.
My best guess is that the model of car is "older" now, and they've subsequently moved onto other interests, including possibly the newer model, which I heard (but can't confirm) this person built a similar site for.
The only real changes to the forum in the past 3-4 years are more ads and many more performance/reliability problems, plus bots.
They might just view it as old, passive income now. If it's a niche and not likely that profitable, you could ask about buying or profit sharing and work on improving and maintaining it. Chances are, if it's not a priority for them, they're well aware it could use improvement and maintenance, but just never get around to it. Yet nor do they want to give it up.
I've run a forum for 20+ years and always wish I had time to do it justice, but other projects generally have better prospects, are more interesting or are more profitable. I'd at least be curious about a pitch that gave it fresh eyes but without losing the entire revenue stream or risking backend or inviting legal issues. In case any of that applies here and is useful to you.
I believe others have definitely reached out to the owner previously just for help with the performance/reliability problems. Not 100% sure on whether they've offered to buy the site or not. Either way everything I've heard is the owner either isn't responsive at all or very much doesn't have the desire to dig into any of the issues.
My guess is also probably the passive income side being the priority, especially since they've definitely increased the ads at least since I first joined the forum about a decade ago.
Thanks for the input. It's appreciated. Unfortunately as you can probably understand it's a relatively simple but complicated problem.
Ads don't make much and forums can be a bit of a pain, so the owner is likely just trying to recoup something for the historic hassles. If monetisation is not ads, it's subscriptions and a site owner can be reluctant to foist costs on regular users (who've contributed a lot of content).
Their reluctance to fix issues could be lack of time or wariness of breaking something and creating more work. The reluctance to sell could be because they don't know how to price it and don't want to regret selling it. In many instances, a web property is something you've dreamed of making bigger and you'd hate to give it up only to watch someone else take it further.
Scraping might just invite a legal hassle. I gather that this is the not the route you're interested in, but personally I'd suggest trying to find a casual way to trade messages with the owner, find out their pain points and go from there. There would likely be a path where they can maintain some control but get help and relinquish the stranglehold.
As far as who owns the data, everything mentioned above is community-owned at least. So if anything it'd be up to individual community members, I'd think, to be OK with it. Since the forum is fairly dead, many of those original members aren't even active either.