If you are reading this, you probably already know YouTube recently removed the Collections feature that allowed you to group your YouTube subscriptions into "folders" with other subscriptions of the same nature. I can only speculate as to why they might have done this, but I think it is probably purely due to what they see as cost-effectiveness. I think they probably found a very small percentage of YouTube users actually bothered with this feature, so they decided to discontinue it and dedicate more of their resources to features most people actually do use.
But, I could be wrong, and YouTube as far as I know has never given an official straight answer to this. Their site merely says "The Collections feature was removed on May 26, 2015. We'll continue to focus on other efforts to make your subscriptions even more enjoyable." I have to wonder if they even saw the irony in this unhelpful statement. ". . .even more enjoyable" indeed.
Those of us who relied on this feature are now left with out subscriptions in complete disarray, and while there are other off-site services that offer ways to organize our YouTube subscriptions, I for one don't want to have to use them. I have seen many complaints that they don't actually work, and who can say how long they will be around, or how well they will be maintained vis-a-vis YouTube's constantly changing UI?
So what are we to do? Well, the simplest solution for me was to create a "YouTube" folder in my bookmarks (I put mine up top in my Toolbar) and then to add sub-folders named for my collection categories. After that, all that was required was to go into "Manage Subscriptions" from my main subscriptions feed page, and one by one open each channel, bookmark it, and put the bookmark in the appropriate folder.
This may seem a lot of work if you have a lot of subscriptions, (I have 173 as of this writing) but it is something that need only be done once, and once done is far easier to maintain and update than the Collections feature ever was; I always found the feature to be more than a little kludgey; far more of a pain than it seemed it really needed to be. Also, for navigation, bookmarks are much easier and readily accessible than the old Collections sidebar ever was. I often found scrolling through it and actually finding what I was looking for rather difficult, especially when my mouse strayed just that little bit and the whole damned thing closed on me. In retrospect, I'm actually kind of glad YouTube did this; I'm much happier with my new arrangement.