All surfaces receiving shotcrete should be above freezing. The mandatory requirements of ACI 506.2-13, “Specification for Shotcrete,” specifies:
“3.4.5 Cold weather shotcreting—Unless otherwise specified, shooting may proceed when ambient temperature is 40°F and rising. Stop shooting when ambient temperature is 40°F and falling, unless measures are taken to protect the shotcrete. Shotcrete material temperature, when shot, shall not be less than 50ºF. Do not place shotcrete against frozen surfaces.”
Your question then asks about substrate temperatures for sections not requiring bond. The concern of frozen concrete is not only bond, but an issue with freezing of some thickness of the concrete that would prevent strength gain. For requirements on this, ACI 301-16, “Specifications for Structural Concrete,” would likely apply and 188.8.131.52(b) requires:
“184.108.40.206(b) Cold weather—Concrete temperatures at delivery shall meet the requirements of 220.127.116.11. Do not place concrete in contact with surfaces less than 35°F. Unless otherwise specified, this requirement shall not apply to reinforcing steel.”
There is a discrepancy between ACI 301 and ACI 506.2. The ACI 301 value (35°F) is somewhat more conservative, though ACI 506.2 provisions (32°F) have proven to produce quality shotcrete. You may consider asking the Engineer of Record for your project what minimum substrate temperature is acceptable on your specific job.