Here is an article that tries to explain how these animated emoticons are added in Subject lines for Gmail.
It says that emoticons are inserted via Unicode (Base64) in the email Subject lines. But in the email I received it didn't contain anything that would indicate the animated emoticons all I see is [?] instead of the animated emoticons in the raw headers.
I even asked the writer of the article cited above about how the spammers are adding the animated emoticons in my case. But even he had no idea about it.
Here are the email headers of a spam email I recently received which had these animated emoticons in the subject line:
https://ctrlv.it/id/67486/4103314734(personal info redacted) [You can notice the question marks that appear instead of the animated emoticons in the subject line]
Here is how the email Subject line appears in the Gmail web client on Chrome:
Can anyone tell me as to how the spammers are including these animated emoticons in the subject lines? And why they are allowed to do so?