should we include or exclude a variable in a logistic regression based on the description below? should we include or exclude a variable in a logit regr. model which will only obtain values if a certain event takes place otherwise will show N/A? this variable tells whether or not a product will be bought based on calls made by the company. the variable tells: number of days that passed by after the client was last contacted from a previous campaign. the values (-1) are for users who
You may want to add an indicator variable, 0 if the event has happened and 1 if it has not.  In a linear model, the coefficient on the original can fit to the "real" slope, while the coefficient on the indicator "fixes" for the missing values (and whatever imputation you use).

https://stats.stackexchange.com/a/105258/232706
https://datascience.stackexchange.com/a/64709/55122
https://datascience.stackexchange.com/a/69841/55122