Since WordPress 3.0, released in June 2010, the themes bundled with WordPress core have been named twentysomething, starting from twentyten.
The latest is Twenty Twenty ( twentytwenty ), released with WordPress 5.3, ready for 2020.
2019 
2020
Here’s a table of the themes bundled into WordPress core by version.
| WordPress | 2020 | 2019 | 2017 | 2016 | 2015 | 2014 | 2013 | 2012 | 2011 | 2010 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 5.3 | |||||||||||
| 5.2 | |||||||||||
| 5.1 | |||||||||||
| 5.0 | |||||||||||
| 4.9 | |||||||||||
| 4.8 | |||||||||||
| 4.7 | |||||||||||
| 4.6 | |||||||||||
| 4.5 | |||||||||||
| 4.4 | |||||||||||
| 4.3 | |||||||||||
| 4.2 | |||||||||||
| 4.1 | |||||||||||
| 4.0 | |||||||||||
| 3.9 | |||||||||||
| 3.8 | |||||||||||
| 3.7 | |||||||||||
| 3.6 | |||||||||||
| 3.5 | |||||||||||
| 3.4 | |||||||||||
| 3.3 | |||||||||||
| 3.2 | |||||||||||
| 3.1 | |||||||||||
| 3.0 |
I’m in the process of updating my dynamic API reference for the themes that are still delivered in WordPress core. So that’s 2020, 2019, 2017 and 2016. Note: There wasn’t a default theme for 2018.
See wp-a2z.org.
Note: The main SVN repositories contain all of the above themes.
For more info on WordPress source see wordpress.org/download/source