Switched lug jacks. They come in mono or stereo. For an fx loop, mono will do: They have two lugs for ground and two for tip/hot. One of the lugs in each pair is switched, i.e. when a jack is inserted, the switched lug is disconnected from the other unswitched lug. If you wire the the send and return to the unswitched tip lugs and connect the two switched hot/tip lugs on the send and return jack together, when there is no jack inserted, the send tip connects directly to the return tip; When a jack is inserted, this connection is broken and the loop works normally. You can just use the unswitched lugs connected to ground, it will make no difference.
Here's an own frame mono switched jack, you can see how the contact between the switch lug and the tip is broken as the tip gets pushed back when a plug is inserted.
https://robrobinette.com/images/Guitar/5E3P_Build/Switched_Jack.jpg