You will be able to add and see emoji manually, regardless of what the plugin can do. Emoji support is a small part of the greater character-set improvements to WordPress as a whole. WordPress is now capable of storing 4-byte characters in the database, including emoji. Additionally, most browsers are capable of displaying emoji natively.
What the plugin removes is the additional checks added to support emoji specifically. There is a detection script added to every page to detect browser support for emoji, and additional scripts to do inline replacements of them with the twemoji character set. These basically colorize the emoji instead of displaying the browser based versions. See here for an example:
https://twitter.github.io/twemoji/preview.html
There is also additional code to support emoji in emails, in RSS feeds, and an additional inline CSS style added for them. The plugin disables that additional code.
But if you are posting from a mobile device or the on-screen keyboard, you will still be able to insert emoji, and you will still be able to see them in the browser. That can’t be changed by a plugin. You were always able to insert emoji into posts, and now WP can store them in the database correctly. And your browser supports them natively, without the twemoji script.
