Dashlane Password Manager's PR team recently explained how the team has tuned the app to take advantage of Android P's treatment of the accessibility service being needed by some apps for core functionality, and has thus shed light on the changes to the accessibility services that happened between Android 8.0 (Oreo) and Android P. The function is essentially turned on for apps by default if an app uses it in a functionally compliant manner as described by recent policy changes from Google. Essentially, the Dashlane team plans to remove the ability to turn on accessibility features manually, and will use them in the app by default from Android P onward.
Since you still have to flip the switch for accessibility access manually on a per-app basis in Android Oreo and older, Dashlane users running those older versions will still have that option. A version of Dashlane supporting the new accessibility behavior cannot be released until at least June of this year when Android P is officially released in its final form, so users running the Android P Developer Preview won't see this feature. Once the new version is released, users on the Developer Preview and other old or unofficial versions of Android P will be able to use the new version of Dashlane in the same way as other Android P users. It's worth noting that this change does not help the situation with certain browsers and other apps that Dashlane cannot access, such as Firefox. Such apps are still off limits for Dashlane, meaning that users will have to create and enter in passwords manually.
Password managers like Dashlane still have their uses, especially for mission-critical applications or individuals whose security needs outstrip the average user. For casual use, however, the Google Smart Lock feature seen in some recent devices running Android Oreo and up can both generate and remember passwords inside not just browsers, but a variety of different apps. Google Smart Lock does require some sort of device security, in order to prevent a user's accounts and privacy being compromised by simply losing their device or having it stolen. This new behavior for accessibility does not change the fact that the guidelines for usage have changed, and Google will soon be taking action against apps and developers who refuse to come into compliance.