Upon the creation of a new Federal Employees Retirement System (FERS) in 1987, those newly hired after that date cannot participate in CSRS.
Notably, though, CSRS employees do not participate in Social Security (unless having worked in the private sector beforehand, and then subject to penalties).
The defined benefits of both the CSRS and the FERS systems are paid out of the Civil Service Retirement and Disability Fund, which had a projected balance of $898 billion as of September 30, 2017.
In general, if rehired employees have 5 years of civilian service as of December 31, 1986, they will retain CSRS coverage.
However, if the break in service is greater than 365 days, the employee is also covered under Social Security and will be deemed CSRS Offset.