Yes, Jaffe's Memo is real.
Guttmacher Institute (former Planned Parenthood branch) republished it, along with other documents by Frederick Jaffe on its official website, in the FAQ section:
Who was Frederick Jaffe?
Frederick S. Jaffe was the Guttmacher Institute’s first president
[...] Some anti-choice activists have attempted to falsely link Mr. Jaffe to coercive population control measures by taking out of context parts of a memo he wrote in 1969. However, Mr. Jaffe’s memo merely summarized various population control measures others had proposed at the time; he did not endorse or otherwise condone coercive measures (the full memo is available here).
He was described as:
most outstanding intellectual leader in [..] fertility
Their Google Docs contain the full memo, titled:
PROPOSED MEASURES TO REDUCE FERTILITY BY UNIVERSALITY OR SELECTIVITY OF IMPACT IN THE U.S.
He also sent a letter to Bernard Berelson, with proposition 1 stating:
[...] continued U.S. population growth will inevitably cause deterioration in the quality of life of this and future generations [...]
Which is contradicted by the steep drop in fertility at the time, that had started 12 years ago. During that period, fertility rates fell from 3.6 to 1.7. When Jaffe published his memo, fertility was at 2.4 (zero-growth levels are 2.1):

Deleting the documents
After July 2022, they deleted the FAQ discussing Jaffe's Memo, but you can find the page in the Wayback Machine (among many internet archiving services). This is why you get a 404 when visiting the link directly: https://www.guttmacher.org/guttmacher-institute-faq
Timeline:
- 14 Jan 2017: first backup of the Jaffe Memo
- 1 Jul 2022: last backup
- 8 Sep 2022: "The requested page could not be found." [Guttmacher Institute deleted it from their page]
Over 80 snapshots of the website. Over 4.5 years until they deleted it. Ruling out the possibility of a hacked website.
Is Guttmacher Institute a credible organization?
Yes. US government websites link to it several times.
Search in Google (or other search engines):
site:gov link:guttmacher.org
This will give you all websites that end in .gov
and contain a link that ends in guttmacher.org
(11000 results).
Then you can inspect each link and verify it's not a random website with the purpose of spreading disinformation.
Additionally, credible newspapers and organizations link to it as well.
Finally, their initial name was "Center for Family Planning Program Development: The Technical Assistance Division of Planned Parenthood - World Population" (emphasis mine)
Snapshot of their Google Drive:
(not all document thumbnails are visible)