On some older planes you still see the spaces they used to be, but typically they're welded shut so people don't cram them full of chewing gum etc. But modern seats have super lightweight liftable armrests that don't even have space for an ashtray.

https://thepointsguy.com/news/molon-labe-new-staggered-econo...

The lavatories have them because occasionally some people will smoke there, and you don't want them to drop the cigarette in the paper trash and kill everybody in the plane when it starts a fire (which apparently has happened, in the past). http://www.iasa.com.au/folders/Safety_Issues/RiskManagement/... But if you google a bit there appears to be lots of fires still, when cigarettes are dumped in trash cans.. so either not all planes have ash trays in the lavatories, or some are too stupid to use them, or maybe they want to hide that they smoke by stuffing the cigarette deep down into the paper trash..