Comments not working after Drupal 9 upgrade

[geerlingguy]

When someone tries to leave a comment, they get the dreaded:

The website encountered an unexpected error. Please try again later.

[geerlingguy]

Error: Call to undefined method Drupal\Component\Utility\Unicode::strlen() in _custom_mail_prepare_username() (line 82 of /home/geerlingguy/domains/drupal8/web/modules/custom/custom_mail/custom_mail.module)

From the logs.

[geerlingguy]

Also found it in Markdown module: https://www.drupal.org/project/markdown/issues/3258028