It looks like the "official" Shopify 60 day trial is over. However, there may be a hack (or two) for this. I'm checking 'em out myself, right now.
Cam Gould from Elkfox, a Shopify Plus partner, sez they can set you up for a 60-day trial and beyond, according to a post on Quora from Jan 8 of this new year (2018).
Add the following to the basic Quora URL, and you should reach the correct discussion.
How-do-I-get-a-60-day-free-Shopify-trial
Look for Cam Gould of Elkfox, then follow the links/instructions.
There's also a post by Jeanette Lau, saying she can set you up as a Shopify dev, which also provides an unlimited trial. Good luck!
Sorry the link isn't clickable. I'm a noob here and not allowed to post links...
