Unhighlight last search in 'less'

You can use Alt+u to remove the highlight on last search results.
You can highlight them again with Alt+u, it's a toggle.

Switching off the highlight does not switch off the status column, showing marks on each line containing a match, if the column is enabled using options -J or --status-column or keys -J.

To hide the status column, use -+J.

To show the status column, use -J.

(Technically, Alt+u it's equivalent to ESCu on terminal level - that is why the Alt-key is not mentioned in the man page.)

From man less:

ESC-u

Undo search highlighting. Turn off highlighting of strings matching the current search pattern. If highlighting is already off because of a previous ESC-u command, turn highlighting back on. Any search command will also turn highlighting back on. (Highlighting can also be disabled by toggling the -G option; in that case search commands do not turn highlighting back on.)