Here are the reasons why I think it’s safe to remove this._editor.isInitialized() check:

Manual testing

It's safe to enter readonly mode/set disabled option before the editor has emitted 'init' event.

The lack of this check in different integrations

• Vue
  • https://github.com/tinymce/tinymce-vue/blob/main/src/main/ts/components/Editor.ts#L77
  • https://github.com/tinymce/tinymce-vue/blob/main/src/main/ts/components/Editor.ts#L86
• Svelte
  • https://github.com/tinymce/tinymce-svelte/blob/main/src/main/component/Editor.svelte#L93
• React
  • React integration has this check in place. However I think it's fine in case of react. I don't think someone could change the props value after editor instance has been created, but before it has been initialized as it was for angular. I can't determine if the React integration suffers from the same issue, but even if it does it's probably not as easy to run into as in Angular.

The origin of this check

It was introduced in 2018 and I believe it was just copied from other place of the file "just in case".

730975e

I believe it was copied from:

where it actually makes sense to not set the content before the editor is fully initialized.

TinyMCE code analysis

Both EditorOptions and Mode classes are being constructed inside Editor constructor. Therefore I expect it’s safe to use these.