Thanks for all your answers. I probably wasn’t as explicit as I needed in my question but now that I’ve been through it I want to share my experience.
As of this Summer you can no longer abandon LPR status and more importantly residence at a US embassy or consulate.
You have 2 options
1) mail form I-407 to USCIS
2) abandon your green card at a port of entry
It takes 60 days to process if you mail the form in and your date of abandonment seems to be the date of processing (eg 60 days later). If you happen to be abandoning your green card to avoid the tax complications of remaining a US resident then this is problematic. If you hold your LPR status for 1 day of a calendar year, you are deemed a US resident for tax purposes.
To be certain, I flew to a port of entry at the border so I could cross back in on a tourist visa and went through the I-407 in person with a CBP officer which allows you to end your residency immediately. I’ll admit, most of the CBP officers thought I was crazy and very few had seen this happen before but the officer in charge of special cases like this says it happens maybe once a week.