From reading the appendix of [Bacon,2019][1], he said

> 1. “Is DD wrong?” Not in general. The DD research design—comparing outcomes for groups whose treatment status changes to groups whose
> treatment status does not change—still can be a good idea. The DD
> specification—estimating the coefficient a single post-treatment
> dummy—**is a *bad* idea when your treatment effects vary over time (*get
> bigger with time since treatment*)**. In this case, just summarize your
> findings in a different way—event-study or a linear trend-break, for
> instance.

*The highlighted words are the one I want to focus, can I ask does it mean that if the treatment effect get weaker with time since treatment, so, DD specification is no longer a bad idea? 
Or,in other words, coefficient in a single post-treatment dummy is a bad idea when my treatment effects vary over time, no matter what getting bigger or weaker since treatment?*

  [1]: https://cdn.vanderbilt.edu/vu-my/wp-content/uploads/sites/2318/2019/10/09023516/so_youve_been_told_dd_10_9_2019.pdf