With French Nymphing originally no flyline was used outside the rod tip. So the leaders could be 30’. 10’ of that could be .10mm (7X) and 20’ of that could be .12 (6X) for easier handling.
Then in response to “is this fly fishing” FIPS made a rule that the leader couldn’t be longer than 2x the rod length. As a result of that RÍO made a very thin flyline, to get around the rule really, because you don’t want a weighty flyline when monofilament Nymphing.
Fortunately, unless we are competing, this 2x rod length or less rule doesn’t apply.
I think it depends how we grew up but for me on the reservoirs I would often fish leaders 22’ long with three flies. And have no problem controlling them. The standard rule for beginners in the UK is that the leader should be 1.5x the rod length. Of course there are many times when longer is better and some occasions when shorter is better.
A well/constructed leader (not a FNing leader!) can be cast to unroll using loops and no flyline. Even relatively long ones to around 20’.
When I was 16 I read an article in FFFT magazine I the UK where a chap was recommending 35’ leaders with two weighed Cased Caddis early season. So I went out and tried that. My goodness that was hard work
Cheers, Paul