I would do:
:%s!https://example.com!https://placehold.co/64!g
Or even better:
:%s!\Vhttps://example.com!https://placehold.co/64!g
To avoid to match https://exampleOcom since without the \V flag the . match any character (e.g. O in the example).
More information with :help /magic
The | is a very special character in the Vim command used to chain command. You should not use it a substitute separator (I prefer using ! when not using /).
For pattern separator you can use any character but:
- an alphanumeric character,
\,"or|
More information with :help pattern-separator