Floating analog input of ADS1115
R6 is too low an input impedance as seen by the divider's output impedance and combines with R5 to distort the divider ratio. R2/C2 also distort the divider ratio for the same reason at frequencies where C2's impedance is low. This results in a filter with a time constant that changes as the the divider ratio changes since R2/R4/R5/R6 all combine to make the R in the RC filter.
Add a buffer (opamp voltage follower) to isolate the RC filter from the divider so the time constant stays the same. Then put the protection circuit on the buffer input to protect the buffer and ADC behind it.
The buffer also lets you add a low pull resistor to the output of the buffer without distorting the divider ratio but this won't stop an open input from floating since a buffer still drives and overpowers the pulldown even if its input is floating.
Therefore, to prevent floating inputs, add a pulldown resistor to the buffer input that is large relative to the divider resistance (at least 100x larger than your total divider resistance R4+R5) so it does not distort the divider ratio by much.