This is categorically incorrect. Partial application is very simple and straightforward.
What becomes difficult, is when it’s desired to cascade multiple function calls in sequence. Most of the controversy in #24990 has revolved around how to use _ to satisfy this desire, of building “quick lambdas” (i.e., not partial application), and to do it at the parser level. Unsurprisingly, that’s difficult.
However, as has been explored here (and expanded upon here and here), it works quite handsomely when paired with function composition. The relevant discussion is here.
That depends on how rational the Julia community can be. It seems that circling the wagons to make underscores behave as they do in Chain.jl has a tendency to whip people up into a fervor, making things difficult 
By my proposal, l2norm = {it.^2, sum, sqrt}, and if PR#24990 is accepted, l2norm = {_.^2, sum, sqrt}, so I don’t see where your proposal adds any utility (other than defending your persistent desire to use _ as a stand-in for it).
Then I shall draw your attention back to this: