I've mentioned this before, but I worked at a competitor to booking.com which had similar kinds of notices. While I can't speak to booking.com's implementation

I've mentioned this before, but I worked at a competitor to booking.com which had similar kinds of notices. While I can't speak to booking.com's implementation - the code we were running was actually legitimately implementing these things based on actual data (views/bookings/inventory). One person got particularly angry that we showed a "last room" tag for a hotel, and then when calling the hotel was told there were plenty of rooms available - and cheaper rooms, too. In that particular case it turns out that hotel was restricting the availability of rooms/rates to us, but not disclosing that. As for the fees+taxes thing... that's always been a personal annoyance. I forget all the details, but it's very much a market specific things whether it's better to hide/show taxes/fees/etc. Large portions of people from the US just assume that they are not included in prices, and compare based on that assuming it'll be some percentage higher when actually paying. Showing an all-in price gets you skipped over/ignored for being too expensive.