There was once a time where this was a workable excuse, but today's browsers are actually pretty good about following standards. If you're doing something that works in some browser, but not all, then you're likely exploiting some weird quirk of that browser.
And of course, not testing if it works in x browser is different than actively stopping your webapp from running if it doesn't detect the right browser/os combination.
I don't know what your product is, but I use four different browsers on a daily basis across three operating systems (it would have been six across 2 if not for the win10 privacy debacle, and even more if I had an android), and I want all my SAAS to work on all of them.
Experience says when they don't work everywhere, IT just dumps the service for another SAAS provider and everyone cheers.
I made a saas tool for mac developers. Fixing all the quirks would cost far more than the potential 2% users are worth to me.
Slideshows - you mean even embeded Slideshare ones? Why? It's a good tool to explain certain ideas, and easily shareable one. We had our slides embeded on a TC post, and we used them as a significant source of traffic.