When voting to close a question as off-topic, selecting "This question belongs on another site in the Stack Exchange network" gives only "retrocomputing.meta.stackexchange.com" as an option. Why is this?
We're still in Beta
Beta sites don't have migration paths. It'd be good to have a list of sites we often migrate to for when we graduate (but we're still sorting out our scope in some areas so that's a while away).
If you want to migrate a question somewhere else and it is off-topic here, check to see if it's on-topic on that site and then flag the question for moderator attention. It'd be good if you posted a comment saying "This should be migrated to [panda.se]" so we can informally judge community support for the migration.
This will also be the process to migrate to sites off the implemented migration paths when we've graduated.
This is by design
In my opinion, there is a good reason to keep the list limited or even to have no suggestion at all.
Every Stack Exchange has their own rules regarding to what is a good question, it is not up to us to decide. We can really only judge what is proper here. Therefore, the proper steps to take if a question is not on topic is to close it.
If OP still wants to ask the question, it is up to them to find a proper place. It may not even be a Stack Exchange site - not every question is a good question on this network.
The old adage of "don't migrate crap" is also relevant. A question that is written to work on this website is less likely to work elsewhere, and would probably still have to be rewritten to fit into the format of the site it is migrating to. This adds extra work for the users on the target site. There's a lot of discussion about this on various meta-sites. No one complains about too little migration.
Migration is also a messy process. The details are explained here and there's a lot of steps that can cause confusion for OP and everyone else. Do you really follow the whole flowchart when voting to migrate the question? If you do, will everyone else?
There's rarely a need to migrate. Vote to close, and let OP find the right place and take the responsibility of verifying if it's on topic or if it has duplicates on the target site.