Make Custom Placeholders work in standard emails via templates
As stated in that thread, when using a Custom Placeholders in templates, it works as intended with Sequences.
However, when used in a standard email where you have selected a Template that contains a Custom Placeholder, the email inserts bracketed text (eg [[ MISSING PLACEHOLDER - MY CUSTOM THING HERE ]]) instead of prompting for the text to use to overwrite the placeholders.
Note, this also does not prevent the user from accidentally sending the standard email without first making edits to the bracketed text. You can totally go ahead and send that email with the missing placeholder text there for everyone to see.