Hey hey folks! Someone on Reddit called this out as well, so I did some digging to investigate what has changed. The short answer is that the volumes is perceived lower due to volume normalization.
Now for the longer answer:
Spotify gives us a volume normalization gain value to apply to each track. Earlier this year, we began applying this gain value whenever users use Spotify Connect. This change was needed because it helps with cases like loud advertisements that play between tracks, which can be very apparent and annoying, especially for free users. They might have experienced a jarringly loud ad when all they wanted was to listen to some chill, relaxing music.
Most of the gain values provided by Spotify are negative, which means they lower the volume. That’s why many users perceive this decrease in volume as a drop in quality, which isn’t the case.