After doing some digging (i.e. FAQ for Stack Exchange¹, What is migration and how does it work?², and Can we have an Off-Topic > Belongs on StackOverflow option?³), I'm unable to find any explicitly defined criteria for what it takes to get another site added to our site migration list under the closed vote queue:
Via implied commentary [³], it seems like there is a threshold of posts that need to be migrated to a different SE site before said site is evaluated and potentially added to the list found in the closed vote queue. This feels like a bit of the chicken and egg scenario as the only other way I am aware to have posts migrated is by having the admins do it [²] after raising a flag on the question in particular. I feel migrating posts in this fashion isn't very streamlined, and as a result, questions that should be migrated often just get closed out for other reasons, which in turn reduces the numbers we would reference to justify adding the site.
Is there any minimum post migration threshold we are beholden to before we can get another site added to this list, are we able to just make a logical argument for another site, or is there just an entirely different set of criteria I'm unaware of that we must meet before we can get a new site added?
I'd love to know because, frankly, I feel we're missing the ability to migrate posts to Server Fault as questions that tend not be database or developer related often qualify as a server issue instead.
In response to @Aaron Bertrand's comment:
dba.meta.stackexchange.com/a/1281/1186 - and as added context, there have been 0 migration attempts to Server Fault in the last 90 days. 18 to Stack Overflow, one to here, and one to Super User.
Thanks for the numbers and link to the additional meta question. It still doesn't provide explicit criteria, but it's yet another example of what I suspect is being evaluated.
As a follow-up to this though, I did a quick review of the vote closures I've been a part of that were migrated, and of the 7 I touched, 3 arguably could have gone to Server Fault (1, 2, and
Hibernate does not close idle session [migrated] closed/removed so cannot say for certain). Two of these three were migrated to Stack Overflow, likely because it was the path of least resistance, and the other was migrated to Super User (which in my mind has a lot of overlap with Server Fault, and I know I flagged that question asking it be migrated to one of those sites).
This basically goes to further solidify my point though that we're in the chicken and the egg scenario. We're using loaded numbers to justify why we shouldn't add a site to the list. I suspect if we were to go through all the migrated questions, you would find some that should have been migrated to Server Fault instead of Stack Overflow, but because the UI is designed to streamline migrations to Stack Overflow and not Server Fault, the numbers are really just reflecting the easier migration path available to the general community.
July 18th Update: As it's been about a month since I asked this question and we've only had 9 total votes against both answers (not including down-votes), I figure that in and of itself is enough evidence this feature, if added, would go largely underutilized. I've marked the "status-quo" answer as accepted, but it's ultimately up to the admins on if they want to include these sites or not. Once again, thanks for the various insights into this process.