Paper.js admits to not supporting IE < 8 (bottom of http://paperjs.org/about/). Other browser support is not even listed. I guess this means that it's not usable for most serious end-user apps now.
I'd say that not supporting browsers below IE8 (or even IE8 itself) is reasonable for many projects. I just looked at the stats for a few of our sites and total IE browser share is around 4–7% on them.
If you have a similar situation on your site/application, I think it's perfectly reasonable not to support anything other than modern browsers, but you have to look at your own stats.
