I encountered Grothendieck's resignation letter from Bourbaki along with its English translation not too long ago on the web, but for now it seems it's nowhere to be found. I've scoured through the Grothendieck circle site for the letter, but to no avail.
I have since found the original source. It appeared in the second part of a two-part obituary published in 2016 for the Notices of the AMS: Vol 63, No.4. Though I believe the link provided by @NWR is more convenient for the sole purpose of accessing the letter.
Here are the links to both parts 1 and 2, respectively: