Earlier quoted context omitted.
Citing from Google‘s recent email on account inactivity: ”After a Google Account is deleted, the Gmail address for the deleted account cannot be used again when creating a new Google Account“
For Fastmail, I could not quickly find their policy. However, I do occasionally receive some emails on my fastmail user address which are not my own, which may indicate that Fastmail user addresses become available after unsubscribing. The emails are from Steam, with 2FA login codes; I assume they only send those with prior email verification. With only the email address it is apparently not possible to contact Steam…
https://www.fastmail.help/hc/en-us/articles/1500000279581-Ex...