It's recommended for wear, durability, appearance retention and feel, but not essential to the physical install of the carpet.
If you do go with underlay, put something with some depth down, at least 9mm if possible, anything thinner and you could possibly argue you may as well have straight layed the feltback.
Of course, different types of underlay can again extend life of the carpet etc, depends on your budget (limited I guess if you are looking at feltback on stairs)
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