Carrier Codes of Conduct provide guidance on using their product (text messaging). This guidance informs CTIA (Cellular Telecommunications Industry Association) standards. Combined with US (TCPA) and Canadian (CASL) laws, these outline best practices, limitations, and requirements by which that all P2P and A2P message senders must abide. Here is what they have to say about Footers.

Do I need to include Footers in messages? - Yes! 

Recurring message programs must include the footer "Reply STOP to cancel. Msg & data rates may apply," in welcome messages and in an ongoing fashion with measured frequency.

CTIA Handbook - Exhibit 2, Pg 12:

T-Mobile Code of Conduct - Section 2.10, Pg 12:

Neglecting to include footers in welcome messages, and with measured frequency in ongoing messages, is a compliance violation that can lead to Message Failures, Audits, costly TCPA Disputes, Fines and Penalties. 

While the mobile platform offers flexibility to manage footer needs dynamically, the few characters saved by their removal is not worth the consequences of omitting footers or failing to include them with regularity.