This suggestion is actually misleading, as only paste() and not paste0() is allowed when expression() is used within a plot, such as to generate title or axis labels:

warning: [paste_linter] paste0(...) is better than paste(..., sep = "").
      main = expression(paste("Plateau test ", L[n], ",", L[x], " curves", sep =
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The rationale is that only a certain number of functions and operators are valid inside expression():
https://stat.ethz.ch/R-manual/R-devel/library/grDevices/html/plotmath.html