The longest slur - ever.

[OCTO]

Could't be just written "legato"?
I guess, doing this in notation software requires multiple slurs.
"Opus Clavicembalisticum IX" By Sorabji.

[tisimst]

Actually, I think this is possible in Dorico. You can also do it in LilyPond, not with any default setting, but a little coding of a custom function allows it. I’ve seen it done.

But your point is well taken. “Legato” should capture what this messy slur suggests.

[MalteM]

I think „legato“ wouldn’t cover the voice crossing … I’m not even sure whether the main purpose of that slurs is the legato marking

[OCTO]

I think „legato“ wouldn’t cover the voice crossing … I’m not even sure whether the main purpose of that slurs is the legato marking

Hah!
Maybe than as a tempo: Legatissimo con delicatezza
Or even better as name of the piece.

[Callasmaniac]

Or "il arpeggio legato possibile".

[Anders Hedelin]

At a first glance it looks like the slur is meant mainly for the sixteenth sixtuplets (and triplet), but then it isn't quite consistent. During the first two quarters it wouldn't have had to be placed above the notes at all, and just at the end it should have gone below the notes if following the pattern. Intriguing.

[John Ruggero]

The slur seems to be entirely for the accompanying sextuplets, since the melody has its own slur. Many a nineteeth century composer would have slurred the first few beats and then stopped at an unexpected point to indicate simile since the rest of the slur is obvious. The ideal then was clarity and simplicity.

[Anders Hedelin]

My guess is that the composer intended to draw a 'hypnotic slur'. The character given for the music: 'nostalgic, very smooth and hypnotic'. Maybe there are no real rules for 'hypnotic slurs'. (I haven't found any with Gould.)

[John Ruggero]

It definitely hypnotizes me if I stare at it too long!

[Den]

It definitely hypnotizes me if I stare at it too long!

I confirm this!
That is "S" slur...