To provide some context ... when WebTRIS was first implemented it had data from April 2015 onwards. A project went ahead in 2016 to add historic data from TRADS which provided the historic data you'll see in the MIDAS/TAME/TMU layers.
In 2017 James (you'll have noticed his name on the forum, has moved on) noticed some TRADS data was missing from WebTRIS and a smaller project went ahead to rectify this. Most of the sites went into their respective MIDAS/TAME/TMU layers but there were 2398 sites which weren't so easy to either map, or be confident of their source type. Those sites were moved into the Legacy layer and not given any site type.
The Active and Inactive status is dependant on whether the site is in the current network model. If a site has dropped off the model it'll shift to be Inactive, if it's in the model it's Active. Additionally if a new site becomes active we wait until the next major model deployment before we include it so there's a period of time where the site is recording, but not on WebTRIS.
Agree the active legacy site filter shouldn't be there. I've asked our developer to look at removing it.