Since yahoo writes and "maintains" the email for my ISP, what is provided in the webmail for mine is provided in yahoo's webmail. As linked to above they definitely have forwarding.
Note that when yahoo forwards they still place a copy of the forwarded message in the yahoo account's inbox. There's no option to suppress that and there is no longer any option to flag them as "read" in the yahoo inbox as there once was prior to yahoo switching to their "new" and "improved" (???) webmail.
IMO when all is said and done yahoo is a pretty poor email service. It's one of the reasons I switched to a
real email service like FM. The other being yahoo is a subsidiary of OATH (which is a subsidiary of Verizon). After reading the
OATH terms of service (follow link in section 4 to
Privacy Center, Information You Provide to Us) you might never want to use them for anything private again. It really bugs me (and many others) that my ISP, a completely separate company and competitor to Verizon, forces its users to agree to the OATH TOS simply by using the email.
By the way the only reason I forward email from my ISP to FM is for testing purposes and in case my ISP doesn't use my contact email address for some reason and sends directly to the ISP's email address. That's never happened though. And no one else other than me even knows that ISP email address exists. So I never get anything in it except when I was playing around with the mail fetch vs. forwarding.