I think this would fit well with the above comment. My character is new to the world, and looking for guidance. What would someone have to do to have their SIN revoked? I could keep that in mind, and structure a character around that.
I imagine the event that leads to her having her SIN removed, and the immersion into the new world, should be enough to get us off the ground in terms of conflict/good stuff. We can absolutely work in the other elements you mentioned as we move forward. I also do like the idea of being closed in a city - at least for a while.
So, getting your SIN removed... Okay, the best way to explain it is like this:
Think about us in the modern world. Think about how connected everyone is to the world they live in. We have credit cards which are a way of tracking our purchases. Our Social Security Number (in the U.S.) is used... well, in quite a few places, potentially, at least when concerning the government. Our pictures are in databases thanks to driver's licenses and school IDs and such. Our online purchases get stored, as do the numbers associated with those purchases; our personal information is linked to video game accounts and... and the list goes on.
It's even worse in Shadowrun. In Shadowrun, you need a SIN to get through various security checkpoints within a city. Your picture is captured and checked in with a database of known persons frequently by cameras, hidden and publicly visible, throughout EVERYWHERE you go. Basically, SINs are even more pervasive in Shadowrun than all our digitally stored information in the real world is. Normal people have no sense of what privacy is in Shadowrun. It's almost like 1984 except the Megacorps are often doing the spying as well as the government.
To get rid of your SIN, you need to get rid of
all of that information. The trail that you've left behind over the years has to be removed. And THEN you need to get false trails created to use as fake SINs for you to make progress as a Shadowrunner, or even to buy a carton of milk at the store.
So, I know that was a long explanation, but the answer to how hard it is to get rid of their SIN?
Incredibly hard. You need to hire real experts to destroy it, or else have your face and body 100% changed and hope they don't do blood tests or fingerprint samples to figure out who you are. (There IS technology with that level of change available, note.)
It's also far from impossible to revoke your SIN, but it's definitely an interesting story element to use, and can easily involve being indebted to someone... unsavory, such as a Triad member or <insert other criminal organization here>.
Anyway, want to take this to PMs? I'll talk to you again when I get back from my tabletop day.