Yes, it finally happened. Source Serif Pro was first published in 2014 as open-source, but only with upright styles. Although its authors had a complete matching set of Italics ready to ship, a decision was made to delay their release, and work on improvements so that the design quality be consistent and match with the Roman styles.
The Italics were completely redrawn from scratch, this time more closely associating with their historic source of inspiration — Fournier’s work. Some of the characteristics found in Fournier’s Manuel Typographique original specimen, were introduced in the new styles released today.
Frank Grießhammer (Source Serif Pro’s author) explains today on his post on Typekit Blog why it took 4 years to release italics:
Source Serif itself is a part of a much bigger project: Source Han Serif. Many glyphs (and months of work) were needed, and since CJK writing systems do not traditionally use Italic styles, the emphasis of my work was to expand the Roman styles as far as possible, which manifested in a version 2.0 release in 2017.
Supported character sets
The Italics were launched with Adobe Latin-3 support, just like the first release of the Roman did. Extended character support is already planned, including Greek and Cyrillic italics as well as AL-4 support. Irene Vlachou and Emilios Theofanous were already commissioned to help draw the Greek Italic.
Download Source Serif Pro Italic
Keep an eye on the Google Fonts page for availability in the Google Fonts directory.
UPDATE: The italics were added on FontSquirrel