If a custom logo is not specified, Drupal will search in the active theme for a default logo image file. This has been changed from a PNG file to a SVG file.
Themes that were providing a default
logo.png will need to provide a
logo.svg. PNG logos, or other image formats, can always be used by uploading the logo as a custom logo.
Themers can create extra custom settings in their themes to give support to non-SVG default logos.
Consider that SVGs will not always have height and width dimensions, and that adding an SVG file with no dimensions could affect the styling of your theme.