You can do that for the odd small tool, but you can't launch GreatNewThing™ without having it's own full domain!
Except it never gets any users.
I suppose the plus side to spending months rather than days on projects that never go anywhere is I have fewer domains. Only 7 here. Sigh.
PuTTY is hosted from a user home directory[0]. Maybe having a fancy URL isn’t everything.
[0] https://www.chiark.greenend.org.uk/~sgtatham/putty/
I like to remind myself that Wordle, when it went crazy viral, had a domain that was very unmemorable
https://www.powerlanguage.co.uk/wordle/
Not having the "right" domain is never the blocker