There is no obligation: legal, interpersonal, or otherwise.
Only the license terms that the software is released under matter.
Most FOSS licenses include disclaimers like: "THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE."
If you did not pay the author for support, they do not owe you anything.
If they choose to provide support or fix reported bugs, that is their choice.
The NixOS team can use the code regardless of the developer's wishes if the license allows it.
If the developer chooses to not support the software on NixOS or change the license, that is their choice.
FYI that clause is void in civil law (most of Europe), you cannot waive away responsibility.
Additionally, the license is a contract (license don't exist as a separate category like in the US), the "[...] WHETHER IN AN ACTION OF CONTRACT" is particularly misplaced.
Only the license terms that the software is released under matter.
Most FOSS licenses include disclaimers like: "THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE."
If you did not pay the author for support, they do not owe you anything.
If they choose to provide support or fix reported bugs, that is their choice.
The NixOS team can use the code regardless of the developer's wishes if the license allows it.
If the developer chooses to not support the software on NixOS or change the license, that is their choice.