Is it possible to have a multi-line comments in R?

R Studio (and Eclipse + StatET): Highlight the text and use CTRL+SHIFT+C to comment multiple lines in Windows. For macOS, use command+SHIFT+C.

You can, if you want, use standalone strings for multi-line comments — I've always thought that prettier than if (FALSE) { } blocks. The string will get evaluated and then discarded, so as long as it's not the last line in a function nothing will happen.

"This function takes a value x, and does things and returns things that
 take several lines to explain"
doEverythingOften <- function(x) {
     # Non! Comment it out! We'll just do it once for now.
     "if (x %in% 1:9) {
          doTenEverythings()
     }"
     doEverythingOnce()
     ...
     return(list(
         everythingDone = TRUE, 
         howOftenDone = 1
     ))
}

The main limitation is that when you're commenting stuff out, you've got to watch your quotation marks: if you've got one kind inside, you'll have to use the other kind for the comment; and if you've got something like "strings with 'postrophes" inside that block, then there's no way this method is a good idea. But then there's still the if (FALSE) block.

The other limitation, one that both methods have, is that you can only use such blocks in places where an expression would be syntactically valid - no commenting out parts of lists, say.

Regarding what do in which IDE: I'm a Vim user, and I find NERD Commenter an utterly excellent tool for quickly commenting or uncommenting multiple lines. Very user-friendly, very well-documented.

Lastly, at the R prompt (at least under Linux), there's the lovely Alt-Shift-# to comment the current line. Very nice to put a line 'on hold', if you're working on a one-liner and then realise you need a prep step first.