I'll be responding to the preview portion of your question as I've been more part of that discussion on the mod team - someone else will be along, I'm sure, to discuss the latter half of your comment regarding BWHR.

Not sure what OBS is after a cursory search (save for the streaming platform, and a search for OBS ttrpg store, which actually leads to DriveThru) so I might be missing a couple marks here and apologies if I do! DMsGuild (DMG) is the primary factor to discuss when it comes to the "navigational burden" mentioned in relation to the subreddit, so that is the example I'll be using. In my time as a moderator (~3 months) DMG is the only site that I would personally be keeping an eye on for this ruling, every other site is pretty straightforward.

You discuss
1) Ease of moderation / going into links
2) Difficult to quantify / arbitrary moderation
3) Most people have accounts
4) Why are the moderators deciding and what prompted this change?

I'll reference these points in my response, as follows.

1) We as a mod team already have to check DMG products because even if it meets *their* standards it still needs to meet *our* standards. This is lightening the burden and making those interactions better so that we aren't letting things sit in the queue for multiple days, which sometimes happens for DMG content. DMG in particular has its built-in fullsized preview function that would be sufficient for our purposes.

2) If a user has their content on DMG but no preview or the preview is loading, the content will be removed, but the removal comment will say something to the effect of "resubmit at your leisure once this issue is resolved." Repeat violations likely wouldn't be held against the user, especially if they're repeatedly showing that the preview was on its way.

You also mention arbitrary moderation (takes too long to load, not enough content) and that's not the case. We could definitely shine up the rule a bit to make sure that it's clear what we're looking for, but there was discussion about setting a specific click limit at some point, though that was tabled for a more flexible approach.

As for content amount, that's adjacent to this discussion and is already handled by another rule that's been in place for a long time (1-5 for classes, 1-10 for subclasses, etc.)

3) This is a fair point, though I think GMB -> PDF is still by the internal definition of navigational burden is like half that of DMG login -> purchase -> download.

4) The mod team's discussion of the topic started as a result of the general feedback of the community in CT4. The mod team started an internal discusssion on the topic around February 24 and that concluded rather unanimously on March 4. It's going to be hard to sum of all of that conversation, because there's a lot of back and forth in the discord. I don't know how transparent I should be, but:

Some of the mods were for wholesale disallowing all DMG content (self & link) and suggested directing them to /r/dmsguild (okay, I'll fess up and admit that one was my idea, though there were some who agreed), and others were more even handed. The two groups came to an agreement of the discussion mainly stemming from how there are clear accessibility issues to DMG. We're also trying to make sure that subreddit rules can be fairly applied to all content regardless of host, and at present it's difficult to fairly assess all content through the rules of the subreddit because of the navigational burden of DMG.

A more senior mod might touch on #4 in a bit more clear fashion as I was present for the discussion, but wasn't the one to make the call in the end.