New Shimano wide range cassette 8 speed 11-40 HG400-8

[Titiritero]

Finally Shimano proving a wide range cassette for us retrogrouches!? Not that I'm going 1x, but it would be nice when pulling the kids on the trailer

https://bike.shimano.com/en-EU/product/component/acera-m3000/CS-HG400-8.html

 

[joglo]

The thing is that you won't be able to serve this cassette by an old rear derailleur. Those had been for 32 or 34T biggest cog at max.
With a long b-screw and short chain (1×X) you could get very likely 36T still work.
For 40 and bigger you need a goat link, but this has negative impact on precision on small cogs.
Ideally simply use a proper almost brand new rear derailleur designed for it... Sure, those look all awful on a classic bike.