Would it be possible to be able to set up train length limits and train limits to depots? I have tried to use the output signal icons but they disappear after 5 minutes or so.

Train limits and Train length limits do not apply to Depot stations. This mod only provides a UI to manage the signals. It does not change any functionality within LTN.

Quoted from: https://forums.factorio.com/viewtopic.php?f=214&t=51072

Depot: (ignores Requester & Provider signals)
Stop is Depot - flags this stop as depot (default=0 false)
Depot Priority (optional) - higher priority will be served first (default=0)

Requester & Provider:
- provided items (positive number) and requested items (negative number)
minimum train length (optional) - minimum length of locos + wagon allowed in this station (default=0 no limit)
maximum train length (optional) - maximum length of locos + wagon allowed in this station (default=0 no limit)
limit trains (optional) - number of trains (deliveries) allowed simultaneously at this station (default=0 no limit)