Guitar barre

Not sure I understand exactly when you write: "There used to be an icon in the Lines palette for this but its gone"
In the Palettes line, there is possibly the text line VII--------, which can refer to a barre notation (by adding a B or a C for example, eg BV----- or CV----).
But this text line has always been in this palette, whatever the different versions of MuseScore, and it always is, of course.
So I'm circonspect. An image could probably help on what exactly you're looking for.

EDIT: or perhaps you have unintentionally switched to the Basic workspace, which would explain why you no longer see this text line.
Return to the Advanced workspace first.

Indeed, probably you are in the Basic workspace, You can switch to advanced using the dropdown menu at top right. Or, simply press the "More" button within the lines palette to see the ones that are being hidden. You can then add from there, or just drag that line only to the top section of the palette, if you otherwise prefer the Basic workspace.

I do not find the barre' in the the line palette (either basic or advance)
either (3.6).

I found it here. hope it helps