It's not "especially forwarding", it's only 251-forwarding to
third parties abusing the original MAIL FROM.  That was always
illegal in STD 10, you had to add the route to the MAIL FROM.

RfC 2821 removed the routing from bounces, and that implicitly
obsoleted 251-forwarding.  It's not SPF breaking stuff, quite
the contrary, SPF tries to fix something broken by 2821.

- Frank Ellermann