Attached RFC 7208 Published in April 2014 specifies the discontinuation of SPF as a DNS RR (Resource Record) type and confirms that SPF syntax should now be applied as a DNS RR of type TXT (section 3.1)
3.1. DNS Resource Records SPF records MUST be published as a DNS TXT (type 16) Resource Record (RR) [RFC1035] only.
The character content of the record is encoded as [US-ASCII].
Use of alternative DNS RR types was supported in SPF’s experimental phase but has been discontinued.
In 2003, when SPF was first being developed, the requirements for assignment of a new DNS RR type were considerably more stringent than they are now. Additionally, support for easy deployment of new DNS Kitterman Standards Track [Page 11] RFC 7208 Sender Policy Framework (SPF) April 2014 RR types was not widely deployed in DNS servers and provisioning systems.
As a result, developers of SPF found it easier and more practical to use the TXT RR type for SPF records.
In its review of [RFC4408], the SPFbis working group concluded that its dual RR type transition model was fundamentally flawed since it contained no common RR type that implementers were required to serve and required to check.
Many alternatives were considered to resolve this issue, but ultimately the working group concluded that significant migration to the SPF RR type in the foreseeable future was very unlikely and that the best solution for resolving this interoperability issue was to drop support for the SPF RR type from SPF version 1. See Appendix A of [RFC6686] for further information.
The circumstances surrounding SPF’s initial deployment a decade ago are unique. If a future update to SPF were developed that did not reuse existing SPF records, it could use the SPF RR type. SPF’s use of the TXT RR type for structured data should in no way be taken as precedent for future protocol designers. Further discussion of design considerations when using new DNS RR types can be found in [RFC5507].