A year after this letter appeared in Captain America 206, the Falcon was gone. Jack Kirby's last issue as writer/artist was 214. The Falcon was written out of the main storyline in issue 217 by Roy Thomas. He made a cameo in 218 by Don Glut. He appeared in a filler short story in the back of 220. In issue 223 (written by Steve Gerber) the Falcon was dropped from the title of the series without explanation. I was reading the book at the time as a kid, and I was not a happy camper.
Who made the call and why? I don't know. There may have been a hint in the letters page of issue 222:
Steve [Gerber] feels that most of the supporting players in earlier issues of CA&F-- Sharon Carter, the Agents of Shield, etc.-- have about had their day. He would like to endow the magazine with a new cast of characters, a different type of plotline, and, generally, a whole new look to see it gracefully into the 1980s. Do you agree?
The next issue the Falcon was gone, though Gerber never did get around to giving the series a whole new cast of characters. Roger McKenzie took over the series with issue 227, as the merry-go-round of post-Kirby writers continued.
Later writer Stan Lee hated kid sidekicks though. He killed off Bucky in issue 66 in 1948, replacing him with a new hero named Golden Girl. Bucky's death was ignored when Cap's series was resurrected in the 50s, but Stan killed Bucky off again in Avengers #4 in 1964. After experimenting with teaming up Cap with Rick Jones (the Marvel universe's all purpose sidekick, who also teamed with the Hulk, Captain Marvel and ROM: Spaceknight), Stan created the Falcon, and eventually included him in the title of Cap's new series. The Falcon did a lot to ground the series in present day New York City, and provided Cap with a capable partner who had reason to be a bit more skeptical of America than Steve Rogers. I sorely missed the Falcon's presence in the book when he was dropped.