With the new update to the Text, HTML, and Embedding block you can still embed your custom code/HTML as before, and now you have two ways to do that.
1: Source Code button in the Format Options Bar
Click on the 'Source Code' button with the icon '< >'.
2: Source Code button in the Format Options Bar
Click on the Format option and select Source Code.
Choosing any options from above will open the HTML block where you can embed your custom code easily.
When you write in the Plain Text area, it will automatically generate the HTML code in the HTML block for that.