Cleaning up the bags and banks is something I do before every new expac.
Although, sometimes Blizzard catches us unawares and makes something old useful again. So I often hang onto things that I feel might have a slight chance of having some future use.
But generally all the Shadowlands mats will be either sold on the AH or vendored. I tend to keep things like skins/leathers because they can be useful for making gear for transmog. And certainly all the gear will be disenchanted or vendored.
Like you, I have a crapload of stuff that needs to go.
I used to think this. But leveling is so easy these days it’s not even needed, and I usually forget they’re in my bags at all and just end up vendoring them anyways.
You can start a character naked at the beginning of an expansion by vendoring everything - including the gear they’re wearing - and still level just fine. (I’ve done it, it’s fun.)
True, but it never hurts to have them on hand in case you do get into trouble and need them, especially if you’re playing a class that doesn’t have many healing spells.