I was planning a new post about this but I've been lame, so I'll post it here. I've talked to a couple of the larger online tobacconists, a Mac Baren representative, & an incredibly helpful woman from STG.
So far as I can tell the following only applies to the UK & is related to Brexit.
McConell is the least clear, that appears to be more of a we can't get hold of it. Rattrays looks like it's a gonner for sure. I've heard that independently from two tobacconists.
I looked into Mac Baren much harder because they make my favourite, Bold Kentucky. & they're gone apart from the stuff distributed by STG (St Bruno ready Rubbed, St Bruno Flake, Gold Block, Original Flake, Highland Mixture, Capstan, Three Nuns). One tobacconist thought it might get reintroduced later in limited editions at a much higher price point. That's not the impression I got from Mac Baren or STG (who reached out to Mac Baren as well).