Can PhoneLeash be setup to forward texts only from specific numbers, or with specific keywords?
There is no filtering of any kind in the PhoneLeash app itself, all incoming messages are forwarded to the destination.
If you forward to email, your email service is likely to provide filters. PhoneLeash messages are very well structured, and very compatible with email filtering rules.
Gmail is especially good when it comes to email rules. In Gmail, click on Settings / See all Settings / Filters and Blocked Addresses