Does WinRAR actually expire or is it just a warning?

Don't expect it to actually expire. WinRAR shows you that annoying popup for years, literally. It works the same way, compresses and decompresses files normally, the popup is just a license warning. A lot of people just leave it there and it keeps working indefinitely, so technically it's not a real threat to actually block you.

That said, if that notification window bothers you every time you open it, the license price is reasonable and you only pay it once. Or you can switch to 7-Zip or another open source WinRAR alternative, which do the same job without any popup. It comes down to how much it annoys you seeing it pop up - if you use the program a lot, maybe it's worth buying it to stop seeing it.