Platform: OS X Server · Yosemite 10.10.5+
Update date recorded by Apple; compatibility is version-specific.
Apple source ↗Apple's security archive dates OS X Server 5.0.3 to September 16, 2015, for Yosemite 10.10.5 or later. September 30, the El Capitan client release, is therefore not used as that Server release date. Application and operating-system numbering remained separate.

Apple · Security updates · 2015 archive
On September 24, 2015, Jon Brodkin reported in Ars Technica that Server 5.0 worked with both Yosemite 10.10.5 and El Capitan 10.11 and was a free upgrade for users of Yosemite's Server, while retaining a US $19.99 purchase price. This is contemporary reporting attributed to Brodkin, not a current offer or a free licence for everyone.
Jon Brodkin · Ars Technica · 2015-09-24
Apple's original 5.0.3 security documentation records Apache 2.4.16, BIND 9.9.7 and PostgreSQL 9.3.9 as component versions incorporated to fix vulnerabilities, and removal of Twisted from Wiki Server. These are specific component fixes, not proof that the entire product was free of problems.
Apple · OS X Server 5.0.3 · Security
Apple dates 5.0.15 to October 21, 2015, specifying Yosemite 10.10.5 or El Capitan 10.11.1 or later. Its note documents BIND and Web Service fixes. This entry covers the 2015 5.0.x branch, not the whole 5.x family; exact compatibility stays attached to each update.
Apple · Security updates · 2015 archive · Apple · OS X Server 5.0.15 · Security
Selected releases, not a complete history. Dates follow Apple's release records, not the latest page edit. No builds are included.
ISO date: year-month-day.
Platform: OS X Server · Yosemite 10.10.5+
Update date recorded by Apple; compatibility is version-specific.
Apple source ↗Platform: OS X Server · Yosemite 10.10.5 / El Capitan 10.11.1+
Update date recorded by Apple; compatibility is version-specific.
Apple source ↗