The user profile gets removed after a certain amount of time if it does not sign up. After some digging, user profiles that have not verified their email yet get removed after 2 weeks.
The email_token_valid_hours admin setting determines the time frame a user has to verify their account or reset their password.
If a user does not validate their account within the specified time, the token they received in their email will expire. This means that the link they received to activate their account or reset their password will no longer work, and the user will need to request a new activation email or password reset email. This will generate a new token and restart the verification period as defined by the email_token_valid_hours setting. The default value is 48 hours.
May also be worth knowing about the purge_unactivated_users_grace_period_days setting, which determines the number of days before a user who has not activated their account is deleted.
The default value for this setting is 14 days, so that if a user registers an account and does not activate it within 14 days, the account will be automatically deleted.
However, this setting can be adjusted according to the needs of your forum. If you set it to 0, unactivated users will never be purged.
Note that this setting does not apply to staff - their accounts will not be purged even if they are not activated within the specified grace period.