One of the most unexpectedly fascinating documents in the Barclay Perkins archive at the London Metropolitan Archives contains master copies of the circular letters sent out to their tenants.
Mostly because it covers the war period, when all sorts of strange things were happening. Admittedly, a lot of the letters are requesting, with various levels of implied and unimplied threats, to return empty bottles and crates. Or telling tenants they won't be able to get as much beer as they would like.
But this one documents the death of three of Barclay Perkins beers. Casualties of war.
They did have a ridiculously large draught beer range - 11 in total, of which five were Milds. So it's not surprising that there was some trimming. The reason given "restricted plant" doesn't ring true. Especially as all three discontinued beers were parti-gyled with ones which continued to be brewed.
I suspect the real reasaon for dropping PA was that it was too strong at 1053º to be viable as a product when brewing restrictions were starting to kick in. As they already had a weaker Bitter, XLK, it made more sense just to concentrate on that.
As for the Milds, my guess would be that the discontinued brands weren't great sellers and the brewery reckoned they weren't worth the trouble.