We already support this, somewhat. In subject lines, we don't let you use all types of emojis that you could inside of an email. We only allow you to use text symbols that have default emoji representations of them. For example, if you enter most these symbols ☂ Symbols — CopyPasteCharacter.com in a subject line, the recipient's email client will pick the correct emoji character to represent it. We do this to ensure that the subject line will display well, regardless of email client.
I think this is an unfair categorization though. We can't vote on this idea so you can't see how popular/wanted it is among users and then we create more ideas..which is clutter. Can you open this up again for votes?
This feature is also supported by Hubspot and Pardot.
This is one of 3 ideas asking for the same thing, that should be merged in the ideas portal by Marketo Product management. Because Marketo Product Management has inappropriately marked this feature as "already have it", Marketo users are prevented from voting up this feature request.
Here is a video demonstrating the lack of UTF-8 character set support in Marketo:
We do partially support it, which is why it is marked this way. I've mentioned it in other ideas/threads, but we know that customers want full support and we don't ignore it just because folks can't vote more on this idea. It's not like we have forgotten that customers are interested in full support.
But you are incorrect if you think we don't partially support unicode standard emojis in the subject line. Consider unicode character U+2600 (☀️). Go paste that into a Marketo Email subject line and it will work.