I will also be adding support for WATCH numerics 597, 598, and 599:
:host 597 a b user address 1397555233 :changed away message
:host 598 a b user address 1656402297 :away message
:host 599 a b user address 1656402286 :is no longer away
Unfortunately, it looks like InspIRCd 3 is not covering all of the cases for these events.
For example, if nickname A is on IRC and has set an /away message, if they change to nickname B, and nickname B is on your WATCH list, your client will receive numeric 600 to let it know the nickname is on IRC, as expected. But your client will not receive a message to let it know the nickname is away. You would need to use /notify to update your notify list to receive the new 609 numeric with the away message.
The only way to resolve this would be for InspIRCd 3 to either send numeric 598 after 600, or to use a new numeric instead of 600... unfortunately, a new numeric would break the WATCH list for all existing clients, in the same way as adding numeric 609 to replace 604 did.