As far as I understand it, it's not an expiry date, it's a date until
which the SRFI has to stay in draft status. I used this time to iterate
on design and implementation and to ask for feedback (thanks to all
involved for their suggestions and critique!)
Sixty days is the official deadline, but extension to ninety days is allowed. However, ever since I took over as editor, I've been lax about enforcing those deadlines because life has intervened in many authors' attempts to meet them. Also, some excellent SRFIs have resulted from this flexibility. I will admit that some SRFIs have languished, and I've had to withdraw a few myself, or recruit new authors. However, SRFI 253 has been particularly lively, with may improvements and eight new drafts in its short life, so I'm not worried at all that it's stalling.